Because even though they say that it's backed by a dollar per tether, there's literally nothing that is keeping it from being traded like a speculative security like BitCoin and other cryptos are.
Why would anyone speculate it to be worth more than a $1 though?
Liquidity premium on having your nominal dollar already in crypto for trading; liquidity premium of being able to send stable nominal dollars via crypto mechanisms.
Consider, why do some bonds trade at negative interest rates?