The only real market is Kraken - the rest is trading against USDT, or has no volume, or just not acceptable to be used as exchange
Nothing is preventing you from exchanging USDT for USD.
Can you show proof of ANY USDT->fiat withdrawal? From what I've heard, USDT cash is basically impossible because money won't go in or out of Finex, who owns Tether through a holding company.
I don't need to show any "proof" and what you've "heard" is totally irrelevant. There is at least one very large and reputable exchange that has no problem whatsoever with fiat deposits and withdrawal and where you can trade USDT/USD.