Please provide one reason to believe that USDT is actually 1-for-1 backed by USD. Because they say so?

Tether is the crypto version of a wildcat bank. It's totally unaudited, it's owned and operated by a former plastic surgeon and others with limited experience in finance, and it's been heavily fined by the state of New York for its shenanigans. It's only a matter of time before they implode like 3AC, Celsius, FTX, Signature, etc. etc.