Because by setting an arbitrary 3000 hour minimum, you miss out on the really good applicant who only has 500 hours, and instead hire the dropkick with 3000 hours, and spend the next 30 years trying to uptrain the dropkick, while the 500 hour pilot goes and flies for the opposition.
I did mention career airline, this may not apply to all airline jobs. But at this airline they spend the first few years as an S/O, gaining experience. QF mainline doesn't put 500 hour pilots directly into the RHS of a jet.