This is the issue. No. I do not think that an improv DM can produce the deep and immersive world that I do. I don't believe it for a second. That is the issue here. I'm not conceding this point and we will just have to agree to disagree. You speak in theoreticals but you have admitted that I should be aware it's improv and run with it. Being aware it's improve prima facie will reduce the immersion. There is nothing over the hill. Nothing. I have to run over the hill to make it come into existence. That violated immersion right there. Full stop.
So you are not the arbiter of what is immersive or not. I am not immersed by improv. Let's just agree instead that for some people they can become immersed with different stimuli. If I know certain that the DM is making it up as we go, I will not care a whit about the world. My investment will be nil. In fact, once I figure out what is going on, I will politely bow out of that campaign.
So it is the CERTAINTY you exhibit about these matters that is triggering. You turn an opinion into a scientific fact. Your opinion is yours. It's not a scientific fact. I'm not unique in finding your improv anti-immersive. I'm not claiming my way is the exclusive way but I am say for a lot of people, things like immersion matter in the way they matter to me. It's why they keep using the term. Immersion is a pretty subjective concept.
Of course I'm not the arbiter of what you like -- I've said this multiple times. My point has never been that, if you gave it the chance, you'd find you like improv games -- this would be a silly argument. Instead, my point has been that when the fiction is created is not the point of contention.
Like your opening statement -- if what's over the hill is exactly the same whether improved or prepped, you're saying that your immersion depends not on the level of detail or the fiction presented, but instead on
when it was created. If was it created in the moment, it's non-immersive. If it was created last Wednesday, it's immersive. I absolutely think this is silly, but what I don't think and haven't said, is that there's not something important here! I very much think there is something important in the difference between improv and prep, but when the fiction is created
is not it. So, then, it remains to examine what it is that is important.
Another poster pointed to the feeling of agency -- that improv feels like they have less agency. And, absolutely, this is a great reason, and in a game like D&D, where the GM holds all the authority over the fiction, it's hard to escape a GM making things up as they go as allowing for suitable agency for the players. This absolutely goes to immersion -- it's hard to immerse in something that you feel you have no control over at all. But, I've pointed out that it's not really improv that causes this divide, but rather authority structures when paired with improv. You can have the same problems with agency in a prepped game -- and this is where railroading comes from.
There's also been talk of skilled play, and this can similarly be very different between prep and improv. I'll readily agree that the kinds of skilled play possible in improv are different from those in prepped play, so this can be a large preference difference. It's not that you can't have skilled play in improv games, it's just the kind of play is different. You're playing the engine rather than the key, whereas in prepped games you're playing against both. Playing against the key is definitely an valid play goal!
I mean, I'm currently running a WotC AP, in 5e, with lots of prep. I'm certainly not one-true-waying here at all, because I'm actively playing in a heavy prep game with strong GM control over the fiction and the plot pacing. It's not like I dislike this approach.