It's called a Social Bond, whee a group of people create something & agree about it's value, it's the same thing happening to USDT & every Stable Coin you're seeing popping up out there.
The last big USDT External Audit was a big fail, but people are still trusting it because it's the Number 1 of the Stable Coins. The USDT team still trying to make it look more legitimate as backed with real dollars, but who knows man, don't go all USDT though.