To be a little less flippant, small companies don't have an IT department to setup and maintain a git server ... and more. GitHib is also an issue tracker. It's also a (minimal) static webhost. And it's also a way to run actions on your builds.

All that isn't trivial to setup or maintain, so small folks especially will often find it cheaper and faster to pay M$ for all that.

And once you’re no longer a small company, setting up any server comes with a pile of bureaucracy. As a dev, I don’t want to set up a server when 80% of the work is filling out SOC 2 change management forms and dealing with crowdstrike. (And nobody wants to spend time debugging why crowdstrike makes the CI pipeline slow or arguing with your auditors about why your ephemeral images don’t actually need crowdstrike.)

This makes sense for small companies that are pizza shops and barbers. It does not make sense for small companies that are in any sector of IT.