Because there is standard functionality they all need.
Virtually every type of page has a predefined template to make sure they have everything they need. Every time you create a page by hand, you inevitably do a half-assed job of it, and they must either be repaired by an admin, or deleted outright and recreated. This is not endearing you to the admin team.
We strongly recommend that you use the Page Creator in the Troping Utilities submenu to the left for anything you're tempted to create by hand.
-- Looney Toons, admin
CC: @Labster, @Robkelk, @QuestionableSanity, @Derivative, @SelfCloak, @GethN7