What incentive would they have to develop a callback system

Well they don't develop it, it's a feature of whatever PBX/VoIP system they went with.

and use it, but delay the callbacks?

It wouldn't be the first nonsensical business decision that businesses latch onto for reasons. You could ask 10 different call center employees and get 10 different reasons why. I've personally heard a few different reasons from inept managers, call center staff being paid min wage so they don't care to people on the call back list get placed in a lower priority queue