There's a question of what rate these traders are going to do this arbitrage in a bank run situation. To do the trade, they need to pay cash under the assumption they'll be able to get it back later. (How long? A day? More?) Their risk of losing the money might be low on a normal day, but not zero. What if today is the day things get weird?

So any trader is going to have a limit on how much cash they're willing and able to put at risk, based on Tether's promises. This limit may be lowered if things look iffy.

They do profit if they can buy at a lower price. It's only competition that keeps them from doing that.