Because a guy boxes means he's never trained in anything else at all?
That's Manny Pacquiao posing before a muay thai fight.
People need to realize that MMA is governed by a set of rules. There's no stand ups, time out for recovery after fouls .. anything at all can happen. Until there's a study out there where they pit 100 boxers against 100 mixed martial artists in completely different environments, under different state of minds (intoxicated, sober, enraged) and every now and then one sneaks the other, then no one can say for 100% certainty what would happen most of the time.