What I you talking about? Most of fixes are change a couple of numbers in configs or add a couple of lines in code.. Example: AI weapon selection was bugged, and they never switched weapons. I just rewrote the whole logic from scratch, took literally 15 minutes
Of course, there are situations, when the whole system is to be refactored. In that case, you don't want to fix bugs in the current system, you want to fully focus on a new one, and it could take months. I doubt they work on completely new AI, but who knows..