Part of the advantage is a fully hosted service where you don't need to worry about a control plane managing and scheduling jobs.

There's some cost involved there--especially if you're offering hosted runners (you need some capital to buy infra to run jobs)

There's also advantage of limiting how many companies you share your code with. If you're using GitHub, you're already trusting them with your source code so GitHub Actions doesn't require more trust or an additional entity to manage/pay