Outsourcing goes back a long way. AA contracted out a lot of ramp back in the 80's, IIRC.
The prevailing wisdom has become to not outsource your hubs, because they are vital to operations. Hence CLT, PHL and PHX are not likely candidates for outsourcing. And, it's in the contract that way, for what ever that's worth.
Ironically,out-station fleet generally have more qualifications than hub fleet. In a station I'm familiar with, fleet air started, deicied, worked bag room, operations, a/f, pushed, and cleaned. On top of ramp duties.
Go figure.