Definitely liability insurance and the cost of liquor licenses. Also, some states have fairly good programs in place to find bars/restaurants/sports arenas where alcohol is distributed without age verification. Consequently, many of these bars/restaurants/sports areanas have strict policies where their employees must verify age of any patron regardless of how silly it seems. I worked for one boss in MA who would give one warning if he observed you not checking an ID. If he saw it again, you were terminated.

Obviously silly as common sense implies that one would not have to verify the age of someone obviously old enough. But where's the line? Is it looking 40 years old? 30? Alcohol in general in the U.S. is taboo and feared, so that just feeds into additional regulation.