Someone here may know the exact number but I seem to recall from initial training a management pilot telling us that in a typical year we are overstaffed on pilots and are paying pilots who are "sitting at home and not flying" about 70% of the year. Obviously there are months we need a lot of pilots (holidays, summer) and we keep all those pilots (with pay mind you) even in lean months. So I guess the answer to your question is the standard Delta answer that we get to anything else "because that is how we've always done it."