So you don’t understand how airlines make money? It has little to do with spending money on regional partners. I suggest you do some research.

Customers buy tickets to get from A to B with zero disruptions expected. Compass delaying/canceling flights are disruptions leading to disgruntled customers aka lost profit/business. That’s a start for ya. Delta’s business model is a complete contrast to AA and UA, thus buying the weaker regional doesn’t work for them.