Blue
Ravenous Bugblatter Beast of Traal
Exactly! People who are playing the game need additional things at the game level of play. Be it balance against other classes, progression speed matching other classes over levels, enough complexity to entertain a player with just a single character both in-game and with enough freedom at advancement, and other aspects to make it fun as a game.Yes, but that's because the metagame constructs of class and level aren't actually laws binding the fiction. They're tools to provide a game element to character creation.
NPCs in the game world mostly need a statblock for combat, and most of the time that combat is against the PCs. So as a game their needs are to provide appropriate challenges, etc. They serve a different mechanical purpose.
We're had 3ed and 3.5 where PCs and NPCs followed the same basic rules. But not all of the monster types were balanced against each other or against the PC classes, and the extra load for session prep "advancement" plus running complex-enough-to-be-PCs in-game put a large extra load on the DM. Designers have moved away from that, likely because the game aspects were more important for actual play then the everyone-uses-the-exact-same-rule aspects. We can treat those as tested for the eight years from 3e until 4e, and the end result was not keeping them.

