I guess because it's easier to trade USDT for stock tokens in an Ethereum contract. Same reason USDT is on the blockchain and not just a database run by its owners, who are already centralized of course.

There are also a lot of examples of contracts with some centralization that would appear to remove the entire point of using a blockchain, but they're simple enough that the blockchain is actually easier than hosting their own API, dealing with auth, keeping ddos and hackers out, etc.

Because with a database row you don't risk having SEC suing you into oblivion.

Blockchain tokenization is only for high risk negative reward people.