I think, with modern AAA games, you also run into the problem that they’re typically a dozen games in a trenchcoat. So, you might be able to make the shooting gameplay more difficult by just adding more enemies, but you can’t easily do that for the platforming parts or the stealth sidequests.

You’d need to build the game three times over, pretty much, to really offer three different difficulty levels.
And so, yeah, they try to avoid that by giving help to players that struggle and optional challenges for players that can handle a higher difficulty.