By pressuring their opponent, the swarmer is able to create reactions. Thereby they can catch the counterpuncher in the act of counterpunching or mixing it up.

On the the other hand, the counterpuncher has to do much more than create openings. He must also limit his opponent's punch output, and force them to throw leads that are easier to deal with. Thus removing the swarmer's main asset, pressure.

Other than that, what beats a counter puncher? Well, a better counter puncher of course.