Check out the depth chart for USDT/USD pair. There is no buy-side demand at all. Anyone selling Tether would have to eat huge losses by selling their USDT well under $1. It is human nature to not sell at a loss, so no one is selling.

On the other hand, it takes very little effort ($100K) to prop USDT price up. Rumor has it that Tether themselves are propping up the price on Kraken.

Sad part is that once the prop crumbles, everyone in sell-side of the chart will become bagholders because USDT will be worth nearly zero.