I agree with you but I think that the main reason is that such exchanges don't offer a possibility to buy crypto using FIAT, credit cards and so on. I thought it will change when multiple stablecoins popped up lately but this is not the case. To be honest I don't know any decentralized exchange which offers trading pairs with stablecoins.
Crypto Bridge has trading pairs with USDT, but as always decentralized exchanges have very low trading volume, not only in USDT pairs, but also in BTC pairs and others.
Bitcoin is the highest coin that has traded in
BTC/USDT pair (last 24 hours) on Crypto Bridge, but total volume has been around $320. Other coins in USDT pairs have nearly zero volume.
There are few reasons: USDT pairs was added recently on that exchange; and latest volume of crypto market (within recent weeks) is low.
I don't know that other decentralized exchanges have USDT pairs or not, let's check.