In short because most players won't be able to design fun experience for themselves.
That is the best part ...
They dont need to "design" anything, its all there ... designed and prepared, all they need to do is just follow the rules.

Or even better ... dont search for ways to bypass the rules.

Except that's not the case. Transition from exploration phase into combat is a core mechanic and a key rule - and at this moment it has a lot of issues and imbalances. No one is exploiting bugs, it's just that the ruleset set in the game doesn't work very well.
I said it many times but I will say it again: I don't pay £50 for a game, to then have to sit down and design it. I expect it to be a designed for me, with a ruleset that works and is fun to play. Exploits as they are in BG3 are hardly exploits they are broken mechanics. Stealth in BG3 is broken. It doesn't work. One either doesn't use it, or will find that it trivilizes combat.
Cute ...
But those other people payd exactly the same £50 ... and your money is not supperior to theirs, so ...
Sure, that's I prefer to discuss mechanics from the perspective of how they serve the game, what playstyles they encourage and if they benefit or detract from other parts of the game, how balanced certain tactics are, rather then throwing "me like/me don't like".