Simple: Dany's advisors don't want her to assassinate Cersei.

By the time that a plan like that could be put into action, she is left with just two remaining advisers - Tyrion and Varys, the first of whom would like to see Cersei survive (hence him freeing Jaime and providing them with an escape route) and the second of whom no longer wishes to see Dany sit on the Iron Throne an account of her being... well, a Targaryen through and through. Having Cersei assassinated would help Dany achieve her goals, yes - but by this point, it would not help her advisers achieve theirs, and as such they do not suggest it.

I've been surrounded by people who talk about it and, having read a few articles, I've been amazed at the people who seem to think this is totally out of character. Maybe it has been up until this point (I don't watch the show), but I think everyone who thinks that misses a couple of key points in general.

GoT is a show about murder. Mass murder isn't that big of a stretch

Seriously, the show's most viral moments have all been about murder. Lots of murder. Some people think it's a big deal when a character makes a jump to mass murder, but the larger jump is to get to the point of murdering people in general. To quote Jean Rostand

The death of one man is a tragedy, the death of millions is a statistic.

Kill Cersei and it's tragic. Kill her town and...

In a power struggle, you make a bigger statement by wiping out a civilian population

In Babylon 5 The Deconstruction of Falling Stars (S4E22) they had this scene, which seems rather apt for this GoT episode (emphasis mine)

GARIBALDI: Now, you're a realfact kind of guy. You know what they're up to out there, don't you? Come on, you can tell me. Am I right? Your boys are planning a first strike against the enemy, aren't they? That's why you're rushing ahead like this, because you're under the gun.

DANNY: We will strike at the outer-world colonies and the enemy nations here on Earth at the same time. Our attack fleet will be leaving within the hour.

GARIBALDI: Good. Good, that's very smart. Now, will you be targeting military bases or civilian population centers?

DANNY: Civilian population centers. We, my superiors, feel it will demoralize the enemy. Force a surrender. Estimated dead? Fifteen to 20 million enemy casualties. Can't make an omelet without breaking a few eggs.

GARIBALDI: Absolutely.

So Daenerys wants to be queen. Cersei is someone at least impeding that goal. Oh, and Daenerys has a nuclear bomb dragon and Cersei doesn't. Kill Cersei, however, and someone will simply rise to take her place (Season 9! Oh, wait...) and you still have to contend with her factions (which, the show is ending so you can't have that). Wipe out her city (civilian population center) and you demoralize anyone who would oppose you. It might be sloppy storytelling to have her make that jump in one episode, but it would make sense, in the episodes yet to come, why people might be reluctant to cross her.

That doesn't fit very well with Dany's character. She's always been more the type to go in dragons blazing. Even if it did, it would win her no favours in a land where she's already seen as a usurper and a foreigner.

It wouldn't be wise for her advisors to suggest assassination when they want to give her the best chance at ruling with good favour from her subjects. A good way to win their respect is to defeat the current ruler in open battle.

I think it's also good to mention that it's not what we as veiwers wanted. Everyone wanted to see Dany fly over King's Landing with Drogon and unleash her fiery vengence - and that's exactly what we got.

Lets think about this in simple terms. Daenerys' goal here is to take and hold the throne. People have to accept that it belongs to her.

Now if a ruler is murdered, who is the new ruler? Well, whoever is next in the line of succession, that's who. Probably someone as closely related to the murdered ruler as possible. But whoever that is, its almost certainly not Dany.

However, if a ruler is removed from power by an army, once all fighting has ceased, who is the new ruler now? Clearly whoever the commander of that army says is the new ruler. That's the only way Dany is going to get that throne.

So while the resulting confusion of a sneak regicide might be militarily useful, it won't change the fact that Dany won't be able to rest herself on that throne without going there with an army and taking it by force.

There's also of course the old saying I made up: Don't fight Killer Croc in the water. Subterfuge and assassination is Cersei's forte'. She's really good at using and surviving that kind of attack. For all Dany knows, she'd notice and avoid that easily, and assasins bound for Dany are on the way if she gives them time they need to work.

The control and use of military force is Dany's strength. In her mind, that's the real source of all her power. She's certain she can win that way, and it will show the world that the throne belongs to her.

Daenerys' access to assassins isn't clear, and may not be very good. And she has reasons to be skeptical of their effectiveness.

Daenerys has never really used assassins. Specific reasons aren't exactly given, though I would assert that she is usually contending primarily against groups and ingrained social customs, which aren't really subject to a knife in the dark. She used other means to accomplish her goals instead. Her only personal interaction with assassins has been as a target, and notably, those efforts all failed (not a great confidence-builder in the approach).

An assassination plot takes time, skill and luck. She's not going to charge Joe Random with infiltrating the palace and killing the Queen, who is under personal guard at nearly all times. She could send an experienced soldier, but that doesn't necessarily indicate the right skill set for infiltration and quiet elimination of enemies.

She has, for large portions of the story, not had access to the resources to hire a top-tier assassin, and while she almost certainly does by the time she reaches King's Landing the best assassins we know of in the setting all come from Essos and are groups with whom she has no real association. Even assuming she has the right contacts to immediately engage such services (a big assumption), it would take months to send a message to Essos and contract with a Faceless Man or a Sorrowful Man. And never mind extra time for negotiations! She consistently displays impatience to take King's Landing, and so may not be excited to wait extra months for a potential failure.

Daenerys' advisors might have knowledge of more local options, but no better access. Davos definitely knows about what Melisandre was capable of, but she's not available at that point. Arya would certainly be a good candidate, but she doesn't work for Daenerys, and Daenerys doesn't know her (who would trust such a sensitive task to a stranger of uncertain loyalty?). She doesn't have access to a cunning planner and convenient opportunity (like Littlefinger, Olenna, and Joffrey's wedding).

Finally, Daenerys' successes mostly come from her following the Targaryen archetype. When she tries politics and diplomacy, she tends to not get much of what she wants and faces massive opposition. When she personally destroys her enemies, she tends to get her way.

Note that the Westerosi still believe in trial by combat. Regardless of their (alleged) civility, they still revere those with combat prowess, and the general populace looks down on cloak and dagger approaches.

Taking the throne by forces sends a strong message that Danaerys is the strongest, which incentivizes people to bend the knee to her.

Taking the throne by assassination achieves little to nothing.

First of all, the Targaryens were dethroned and exiled from Westeros. This is a bit of an oversimplification. Robert B actually pressed his claim to the throne as a distant Targaryen relative, but the man in the street knows that the Targaryens have been dethroned and doesn't bother with finicky imprecise rules and statutes.

When Cersei dies, that doesn't automatically make Targaryens accepted again. Most people would still consider her exiled and thus ineligible to inherit the Iron Throne.

Secondly, it's hard to prove that you were the one to orchestrate the assassination. Anyone could claim they did it. The cloak and dagger nature of assassinations very much obscures the assassin.

Furthermore, even if you prove Dany had Cersei killed, there is no rule that killing a king (through assassination) therefore grants you the Iron Throne. Even if Cersei dies, the city of King's Landing will still protect itself, led by an interim regent. Cersei is no military genius, her presence is not significant King's Landing's forces.

Your question seems to be based on the assumption that killing Cersei almost literally opens the door to Dany and her army, which is simply not the case.

But if Dany takes the throne by force, she gets access to the Red Keep to do so, and many people will have witnessed her victory, fear her army, and thus will bend the knee to her, which perpetuates her power.

This is exactly what Robert Baratheon did to claim the throne: he forcible killed/removed all other viable inheritors, proclaimed himself to be the new King, and then actively worked to suppress anyone who would undermine his rule (e.g. by trying to assassinate Dany, which Ned refuses).