Gitlab is open source. You can download it and host it yourself. A decentralized developer community is resilient against this sort of attack for the very reason GitHub is so vulnerable: size.
Um, what? Sorry but if someone is going to send, say, ten million malicious contributions (or heck, even just one), I don’t particularly want to deal with that on my self hosted server. I’d rather someone else deal with it.
Git was always designed with decentralized development and collaboration in mind. Its creator, Linus Torvalds, prefers not to bother with servers like GitHub at all. Git can even be used entirely over email (Linus’s preference)!
The Linux project created Git to solve problems they had. Pretty much no other project in the world has the same set of problems - it’s a highly unusual open source project with tens of millions in market value. Other projects have very different needs.