Git uses, in fact, a "blockchain". There's no mining involved (although, given how hard some code reviews can get I'd argue there is still "proof of work" lol) and anyone can create new blocks and is free to accept or reject them. Like someone else already said I think it's less about the "blockchain" and more about finding ways to decentralize traditionally centralized services.
Yet all our git repos are centralised on github
That's a copy of the repository, not the repository (there is no such thing in a system like git).