The forward assist was a political decision during the AR15 development in Vietnam, Eugene Stoner didn't want it there as it can and will induce a catastrophic malfunction (aka; kaboom) if you force a round that doesn't want to go into the chamber. If it doesn't want to go, that means there's an obstruction. You don't want to force a round into a chamber/bore obstruction. It's supposed need was compounded by the other political decision to not chome line the chamber, and to use old, bad powder for the new cartridge, which compounded meant the chambers were rusting and pitting very heavily in Vietnam.
Stoner had already integrated a sufficient forward assist anyway, by designing a divot into the side of the bolt carrier, in the ejection port that you can press forward with your thumb.
Like many things that don't make sense, the design stuck in the drawings and in military doctrine and that's why it's still made today. But people know it's useless so that's why we've been seeing manufacturers produce upper receivers without a forward assist for a while.