Part of the answer I think is that you can lock up Tether at 12+% interest. So the system encourages withdrawing tethers from the system, at unsustainable savings rates.
https://bitcompare.net/coins/tether/savings-interest-rates
The other part is that it is not easy to directly trade USDT for USD. Kraken is the only place you can do so directly, it is the only place the peg is directly tested. Apart from that to get USD you need to trade to something else, like BTC or ETH, and then sell that for dollars.
The only people likely to have held USDT are also likely to be long crypto, so not surprising they wouldn’t cash out.
The better question is who are selling their BTC/ETH for Tether, and what do they do with the money after? Surely the locked in savings are part of the answer: interest rates on stablecoins are much higher than on BTC/ETH. But I don’t think it is the whole answer.
Presumably enough people don’t question the peg that it can stay afloat for now.