Because regular classes would be more boring and slow. Furthermore, a lot of AP classes at my school don't require people to show up to the lectures (we just need to show up to tests, quizzes, and labs for science courses) and often have little or no homework.
Senior year, when I was taking all APs (my school have a rotating schedule, so we only get 4 - 5 courses a day instead of 7), I get to skip school for a day sometimes:)