I think it's an obvious scam but it's a very useful scam, so there are many people with a vested interest in maintaining the illusion.

From people who want to pump up the price of other cryptocurrencies, to offshore exchanges that want to avoid KYC requirements dealing with real banks, to ordinary users who like being able to transfer USDT... there's many people incentivized to look the other way and pretend that water isn't wet.