If you say that one of something is worth one of the other, you are promising that you have a bottomless supply of the currency on both sides of the peg. This almost never actually happens and whoever is running the peg will inevitably be caught with their pants down. Bankers specifically love breaking pegs and have the means to do so.

If USDT was run by the Federal Reserve nobody would question the system; because then they'd have the capability to issue both the USDT token and the dollars backing it. They would be considered as fungible as dollars in the bank versus dollars in your hand.

that's true, but only because the US (and the Feds) have had a good history of managing the system well (enough at least).

And if that's the case, USDT is basically USD. It's the same reason why the USD is a reserve currency in the world today.

Tether is attempting to tap into that reserve currency trust (ostensibly, to generate profit for themselves).