We've been doing multiple military exercises every year for decades in South Korea. The biggest one of those exercises, Foal Eagle, already took place in April. Cancelling one of them as a gesture of good faith costs us absolutely nothing. Now if Trump ended sanctions, or gave economic aid, that would be a much more substantive act.
Also, that's not the kind of decision that is irreversible. And it actually gives Trump another weapon to use. If NK drags its feet over the next month or so, he can just announce that they are back on.