And there are some good reasons for not self-hosting: price/comfort, built-in tools, decent security without having to do your own, etc.

It's more about control and ownership of your own server and data and this same argument can be made for hosting your own website (price, tools, security) which is maintained by you.

You control everything on the server and deal with downtime yourself or as with the context of schools, companies, open-source projects have 'sys-admins' to do this work.

GitHub does not fit with everyone's requirements to do source-code management on a centralized infrastructure, which is why some users have mirrors to Github from their self-hosted solution. (Google, Mozilla, Apple, Linux kernel developers, etc)

If you can self-host your own website, then you should be able to self-host a Gitlab, Gittea, etc solution.