The reason why Git and blockchains appear similar is because they are both using merkle trees as their underlying data structure. But unlike cryptocurrency blockchains; git doesn’t have a p2p trustless consensus mechanism.
In a blockchain implementation, every block is verified independently multiple times before it is added to the blockchain. On the other hand, many git projects do not require independent verification and, when they do, they only require one person to sign off on a change before it is committed to the repository. Hence, with at most one point of validation that you must trust, git breaks one of the core tenets of blockchain technology.
You can rewrite git history. In blockchains, the ledger is an immutable history.