Because those companies incentivize it. For one, you can't use an Oauth integration on your app if you deploy in on the Apple App Store unless you offer Apple as an option, so there's a vector for Apple Oauth hegemony right there. And for Google, it really just makes sense for most users - it's an ethic-neutral option that you know everyone already has and uses daily. Facebook is still a popular Oauth option as well, but has lost a lot of collective conscious trust, so a lot of services don't play ball with supporting them.

It's really as easy as these companies that support Oauth incentivizing third-party devs to use them.