Okay.
What do you call it, then, when the DM...does that? When they engage in illusionism--making it seem as though the players are the ones running the show, when actually it's just the DM?
Because that's a thing. People have talked about it on here extensively. I can dig up at least one entire thread about it, if you like.
I would call it bad DMing, which sadly does exist. Fortunately in my experience it's been a very small minority. Quite frequently GMs run linear campaigns where we know that we have to find the McGuffin to kill the big bad or we're all DOOMED but exactly how we get the McGuffin, or if we find a different way to stop the big bad, is up to us.
Nope! Wrong. Incorrect.
My point isn't that this is anything like what the world is.
I am asking--I have been asking, for quite some time now--for the alleged constraints/limitations/fnord that you and others have repeatedly asserted are the thing that mean the GM's choices are not driven by what they choose to happen, but rather in some way "forced". I believe that was a word used to describe it--that the GM has no choice but to do X instead of Y, even though they might like to do Y, because of something that gets in the way. No matter what they might do, no matter how much effort they might expend, they'll just never be able to do Y because something forces them to do X.
Thus far, every single answer has been built on...a thing the GM has complete and absolute control over and which is not in any way separate from them. Context? GM decides what counts as context and what doesn't. Setting? That's literally something they wrote, or something they're re-writing from someone else's work. (E.g. I don't imagine that @Maxperson would merely accept it even if Ed Greenwood himself declared that, say, an alien species colonized a sparsely-inhabited portion of the Realms--nor would I expect him to never ever deviate from, say, the monster design of trolls to say that this troll is weak to lightning and cold rather than acid and fire, merely because--I am making this up, to be clear--all trolls in FR are weak to acid and fire.)
Players can declare actions, yes. But those actions can only be declared within a context entirely developed by the GM--and the consequences of those actions are, likewise, entirely within the GM's purview to control. It doesn't require outlandish things for the GM to still be fundamentally in control here.
You build the world, populate it, make all decisions for all sapient and non-sapient beings, develop new parts of the world when you feel like doing so, decide what information the players will be allowed to learn or forbidden from learning, set the terms for any action they take, create and enforce all consequences of any action they take, and have complete control over what qualifies as relevant context for every single one of the foregoing things.
Yes. Characters take actions. You're still responsible for both 100% of the inputs that go into those actions, and 100% of the outputs that result from those actions. Because it all has to come out of the black box before players are even potentially capable of learning anything about it--and thus of doing anything about it. You cannot act when you have genuinely zero information.
There is nothing I can say that will satisfy you, because it's all been said numerous times. I create the fiction for a world, add inhabitants, throw in a double dash of conflict and set the players loose. I do my best to have the inhabitants react to what the characters have done in a way that is logical but of course it is always a judgement call. Meanwhile the players decide what interests them which may include one of the multiple plot hooks I've exposed or could be something they just come up with on their own. The players always have as much information to make decisions as I think would be reasonable for the characters to have. I have complete control over all of the NPCs and monsters but I do not have any control over what they character say, do, or where they choose to go in the fictional world.
D&D and related games are not for you if all of that bothers you, because that's how the game has worked for the past half century and continues to work for me and the people I game with.