Most, not all, of the folks using gapping, string walking, etc. for target shooting are using fairly light weight bows. Many (not all) whose primary archery activity is hunting use heavier weight bows. Draw weight differences are probably what most distinguishes target shooting from hunting, though those difference seems to be moderating in recent times.
Regardless, the heavier the bow, the more difficult it is to hold while using any aiming system, including sights. Not impossible, just more difficult.
Mostly though, sights have just fallen out of favor. Back in the late 50s, 60s, and early 70s, we used sights quite a bit. In fact most everyone did. Then along came the compound and near demise of traditional bows. As folks began to pick up the stick again, they just eschewed sights. Maybe part of it was a reaction to the developing high tech wheelie bows.
I'm toying with the idea of putting a sight on one of my bows again. Problem now is there aren't very many of them around. Can sometimes find old ones on *****. Sights made for wheelie bows don't fit most stick bows.