Corruption. They don’t trust their own people with data.

Blockchain adds a level of certainty that history hasn’t been changed, and you can at least audit any undo-type actions; it’s the same reason certificate transparency logs and git are built on blockchain-esque technology.

`git push -f` can absolutely do the type of "history changing" that you claim it can't though. CT logs are also quite vulnerable to attacks where the root of the certificate chain gets compromised, for example because they are being threatened by their government. Even blockchains are only protected from history rewriting as long as no 51% attack takes place, and all but the largest blockchains are quite vulnerable to this.

I question this idea of history writing and it being a huge problem.

How far down the chain could a 51% blockchain rewrite occur before consuming all of the computation power of the universe?

I have seen arguments in which the 51% steals all of the new coins but how long can that be maintained, the other 49% needs to pay their bills and audits would inevitably out the bad players. Eventually, the last bitcoin will be minted, making the whole issue mute.

51% attacks don't rewrite history, they rewrite accepted history (with accepted further narrowly defined to mean "accepted for transactional purposes"). It's not like the other 49% are under any sort of obligation to delete the alternative chain from their hard drives.

Additionally a bit flip in a CT (cert transparency) log rendered it invalid and dead. This cannot happen in a peer to peer consensus protocol as the bad block would have been rejected by the working nodes. CT is good tech but it superficially is a private issued blockchain without most of the benefits of public peer to peer electronic cash and digital asset systems.

So when does something stop being “git” and start being “blockchain technology”? When the latter sounds better to investors?

When there are multiple actors incentivized to ignore any history-rebasing. Of course if you're running your own git repo, you know your githashes will change if you go back in history and change something, and it doesn't matter. When multiple people want to ensure that history hasn't changed, they can compare their hashes with yours, and have good evidence of who tampered with the history and ignore them / prosecute them.