I suppose the majority of those people use CEXs; they are faster and a lot cheaper. eXch is a whole different matter, where privacy holds an order of magnitude more value than cost. This means the use case of USDT on any DEX is very weak. I suppose anyone who wants to go from BTC/LTC/XMR to a stablecoin would rather use DAI.
they are faster and cheaper, but they are not kyc free, and a lot of people don't have access to CEXs for various reasons. so exchanges like eXch are their only option.
if we check the daily USDT trading volumes on eXch, it's not small. and not everyone uses eXch because they care about their privacy, they just use it because it's a good exchange.