I just came across the topic "Tether" and was wondering how they can reach such a high trading volumes with their limited supply.

XBT, ETH, XRP and most of the other meaningful cryptocurrencies have trading volumes of below 10% of their total supply per day even when the market is going crazy.
USDT has a trading volume of close to 300% of the total supply per day at the moment. How is this even possible?

I also checked the "rich list" of Tether here: https://wallet.tether.to/richlist It states that "OKEx" has about 8 M USDT in their wallet for example. Regarding to coinmarketcap they do have trading volumes of >2.000 M USDT per day at the moment. So if people would start to cash out their USDT they could never fulfil these withdrawals.

Maybe I didn't fully understand the concept of USDT and how exchanges are using it but if so it would be nice if someone could explain this to me :)