Thotas
First Post
(Full disclosure: I am a Paizo employee.)
1) If Paizo only uses "core books and the adventure" for its APs, then (a) that's a lot of really cool classes, spells, magic items, and monsters from other sources that are off-limits for use in an adventure, and (b) people who own those other sources will feel ripped off that all the cool stuff in the other books they bought aren't being used/supported.
2) If Paizo reprints information from other books (such as a spell, monster, or magic item), that's less new adventure content you're getting with your adventure. For example, the hippopotamus appears in Bestiary 2. If an AP has an encounter with a hippo, and Paizo includes the hippo's stat block in the adventure, that's half a page spent on a hippo stat block—half a page that could have been spent giving more background on the adventure, describing the motivations of a villain, or statting out a new magic item found in the AP.
3) As others have pointed out, the rules from Paizo's core books are online for free at Paizo's PRD. And all other OGL rules content appears on fan-created websites. So if you don't have Bestiary 2, or Mythic Monsters Revisited, or whatever, you still have easy (and legal) access to that supplementary content. Which means Paizo can include a three-line pointer to that content and get on with the new material for the AP.
Food for thought.
That's been an issue in the process of putting out adventures since the beginnings of the hobby -- and one of the reasons that I'm a Paizo fan is that I think you guys have hit the perfect balance for all the reasons you've just explained. Don't change a thing about those policies and procedures.