They used to, for several years, until they were pressured to prove it and could not (27:05).

https://youtu.be/-whuXHSL1Pg

> Bitfinex and Tether recklessly and unlawfully covered-up massive financial losses to keep their scheme going and protect their bottom lines,” said Attorney General James. “Tether’s claims that its virtual currency was fully backed by U.S. dollars at all times was a lie.

https://ag.ny.gov/press-release/2021/attorney-general-james-...