If you do it all, you can optimize the whole supply chain. Maybe you can put some expensive capacity you built to use and leverage it when otherwise impossible, etc.

Maybe it's bad business dealing with lots of non-standardized external hosts, and it drags you down.

Maybe people are abusing the free orchestration to do non-CI stuff and they're compromising legitimate users.

Look, I understand it's frustrating to some consumers. However, it's not irrational from GitHub's point of view.