If I or others say that's not how our games work, do you believe us? That's what I'm finding odd - plenty of people have said in this thread that they don't see games as being structured this way, but that seems to just be rejected?
Like, in the (non-D&D) campaign I was running that just finished up, the premise was 'you're supervillains, set four goals for yourselves of exponentially increasing magnitude - you gain power when you hold progress in your goal, otherwise do as you want'. I didn't prepare an endboss or lots of locations or things like that before the campaign began. I prepared the broad strokes of the setting, enough of the thematics and metaphysics so that I knew how I would add things when I needed to add things, and then basically followed the players' leads.
The campaign before that (which was D&D-based) had a bit more structure but not all that much - characters were each candidates for becoming the avatar of their own myth (or being subsumed by an existing myth if they failed to distinguish themselves), with a bunch of empires each up to their own bit of aggrandizement in various places in the world and a bit of secret lore behind why this stuff about becoming myths was a thing. While the 'mythic ascension is a thing you're involved in' bit was a (wide) bottleneck that would be relevant over the course of the campaign, characters could ascend their myths by doing any number of impactful things or they could go and find the keys to break the whole system - something I dropped a few vague hints about without actually knowing how it would all fit together at the time. In the end, the PCs did pull at those hints, and that *made* me detail those things (as opposed to 'I had detailed those things in advance, and they uncovered them') and as a result the campaign became about fixing the mistakes of the past. I didn't think 'ah, their BBEG fight is going to be against Apophis' when I came up with the campaign, but effectively that sort of was what happened.
The *most* structured campaign I've ever run was a group of circus performers in Planescape climbing the Infinite Stair and going to a different plane every game. I maybe planned 3 sessions in advance at most at any point in that campaign. Around the halfway point of the campaign the players had figured out enough stuff to be dangerous, and all planning basically stopped - it was their decision that the 'destination' there, suggested to them by what they learned on their journey, was to try to directly manipulate belief by going to Pandemonium and using a magical location there to whisper a message to everyone on the planes at once. The BBEG I had originally planned ended up being their ally in distracting and holding off the particular gods who objected to the party's spontaneous 'lets alter reality with subliminal messaging' idea.
Are people not having these kinds of experiences? It seems surreal to me that there's this idea that the DM truly knows anything whatsoever about the last session before the game even starts, and that somehow that's the default expectation.