If I am not wrong USDT on eth network volume is way higher than Tron or any other network simply because those are actually traded inside the exchanges not really used for the transactions such as payments so the liquidity of USDT on ETH network is high even when the fees are high cause it's irrelevant since USDT or in general the stable coins are most often used for trading purpose.
That is why the 24 hr trading volume is more than 50% of total marketcap value of USDT.
either because obviously eth is bigger usage than tron so usdt in it is also has bigger user or because tx fee in eth is too high so people prefer them inside exchange than to be used for transaction
most people that used USDT in ethereum network usually transferring by large amount rarely I see people sending this token by the minimal amount i mean such transaction exists but usually have minimal purpose since sending USDT through ETH means its just wasting money with the fee of around $3 even right now after the dencun upgrade which heavily helps reducing the fee, before it was like $40 just to send USDT.
so if its few thousand dollar I guess it still make sense, but less than that better use l2 or other blockchain with significantly lower fee and I agree large amount of USDT just sitting in the wallet of exchange, most of exchange right now using pair of USDT anyway so these exchange need to have large amount of USDT just in case.
on the other hand I always wondered how many USDT is there circulating in other blockchain compared to the one thats available in ethereum, i'm pretty sure despite USDT also being frequently used in other blockchain the amount of circulating supply of USDT might not even near the amount of whats circulating in ethereum.
right now fortunately so many stablecoin created by smaller foundation exist around various less popular blockchain where there's no USDT contract deployed by tether by overcollateralizing their own version of stablecoin with USDT and few more stablecoin just to bring in stablecoin into their blockchain which helps a lot in some cases but undeniably the risk of depegging is there.