EzekielRaiden
Follower of the Way
Because players do things that cause hit points to be reduced.Yes, the GM can just decide those things, but it is exactly this sort of fiat some people might want to avoid. I mean GM could decide everything, they could also just decide when the enemies are defeated, so why the statblocks need to have hit points?
I'd love to see your examples of player abilities that reduce opponent spell slots. Or interact with any other portion of a monster's statistics other than HP, AC, hit bonus, damage dice/bonuses, and saving throws. Counterspell is a player response to a creature's actions, not something the players can just do whenever.
Saves, AC, and HP, along with hit bonus and damage dice/bonus, are just about the barest of barebones things you need to actually use the system as presented to the players. Now, obviously, you could just choose not to do that. Nobody can stop you. But I strongly suspect most players would not respond well to finding out that all the stuff they've been doing, like attack rolls, inflicting or making saving throws, etc. was completely pointless because you just arbitrarily decided when a creature had expired or dealt a crit or whatever else.