I take your point, and its a good one. The vast majority of engagements still occur at ranges of about 150-300 meters, so current 5.56mm rifles are more than adequate for that role. Designated marksmen and snipers need a different weapon, the 7.62 accurized and with match grade ammunition fills that role.
I believe that most infantrymen serving in Afghanistan would say that weight is their biggest problem. They have even more kit to carry than we did in Vietnam.
Agreed, Aftganistand might be different from most battle spaces. I seem recall reading about Aftgans sitting high in mountain passes and sniping British back in the day of the empire. Apparently they are doing the same again today.