Warning, snarky opinion ahead: because git has terrible cli operations that only make sense to people who under reflog and rebase and the underpinnings of git as opposed to any sort of file-based operations, which is how most people understand computers.

Github looks and behaves like a filesystem with a timeline, which is not accurate to how git works functionally, but that's what makes sense to most people out of the box

Sure, but churning out students on a singular, closed-source, megacorp-owned platform isn't the right thing to do either.

This is a great question and one that I have been asking for quite some time. Many tutorials and guides make the mistake of conflating git with GitHub to the point that many new learners think GitHub is git.

When I teach git, I make it very clear that GitHub is one of many services you could use. For our purposes, we're using it because GitHub is quite popular and is a familiar place to look when interviewing with a potential employer. But I stress that there are other options (eg. I host my own instance of Gitea).