Probably because you don't code yourself

.
Plus if (and it's not even an "if") any mechanism in the game works state wise you're invalidating it if you part a province away from that given state (not even mentionning game balance). The mechanism could only work in a game with dynamic state creation or something like that which is probably too much hassle for the benefit.
No, I don't code myself. That's why I pay money for games, you know.
And in my opinion whether the AI can use single-province peace deals is unimportant, it doesn't need to. The important thing is that I, the player, can use it to create the nation that I want.
If I think that Pilsen is a god-given part of Germany, but not Praque, then I want to be able to take that one province even if it's in the same state as a lot of Czechs. For example. Or if Finland wants Åland back, it doesn't have to take the whole of Svealand to do so (how does this system work for states with a capital in them anyway?).
It doesn't matter whether or not the AI can use the single-province system, because I can. And besides, it's not like the AI can
really use the state-system either. And if someone tells me that it can, well, I'll believe a paradox-AI can do anything only when I see it, not before.