Well, Git's also not a blockchain at least in the way commonly meant by the term. But yeah, it'd be a pretty bad bank account (and an even worse way of doing money transfers.)
git is linear, if multiple users have a different main branch history you have a problem.
Pull requests in github is actually very similar conceptually to a consensus mechanism used in crypto currencies. Everyone has an identical copy of the main branch with an identical history of every commit in order, a PR is saying "I think this commit goes next" and, if you use code reviews, the PR approval is consensus.