its a sport because:
a) you have to physically train at a very high level in order to compete
b) it still requires some athletic effort, posing for hours on stage when dehydrated is exhausting
c) hitting proper poses and perfecting your posing is not much different than, say, properly throwing a football.
example: the ufc scores fighters based on good punches, aggression, octagon control
the IFBB scores bodybuiders on good posing, stage presence, and a well executed posing routine
olympic divers are scored based on execution and bodybuilding is no different
rather than executing a diver into water you're executing a streneous muscule pose onstage
its somewhat different but the general principal of the matter still applies