Its trivial to move from one git server to another as they all do the sane thing and speak the same language. Github acts as a git server but its main value proposition is a collaboration layer on top.

Contributors propose changes (PRs), there is discussion and comments on code, further changes are proposed, etc. It also widely used for tracking issued which is heavily integrated to the PR system.

Many contributors get band and can no longer participate. Projects also sometimes get band.

Its much harder for a project to move the collaboration and issue tracking to another system than the git server.