The US has the capacity to go into space, which is higher than the balloon, and neither you nor I know of everything in the US military equipment.
If they could build the X1 in 1945, a plane capable of reaching the balloon would be a doddle now.
https://en.wikipedia.org/wiki/Bell_X-1
If they don't have that ability, perhaps they should.
IMO if nothing else, this episode has exposed that the US is vulnerable to an attack using balloons, and it should not have been if the thousands of guys in the Pentagon had been doing their job.