This is exactly what keeps it at $1. If someone is willing to sell at $.99 then the number of buyers is gigantic. Ditto with someone buying at $1.01. That’s why it’s news when the peg drops by more than a fraction at a penny.

As for why would someone buy or sell for other than $1.00? Plenty of reasons, if a market opportunity to make 10% in a crypto investment pops up, damn right people will liquidate tether at a slight loss