Because they are just putting on show. Being a bodybuilder at that level is very egoistic. Naturally they don't want to come of like that. Everyone wants to be loved, admired and respected, it's just human nature, oh and did I mention the increasing your popularity and chances of getting fat contracts? In bodybuilding You come first at all times...
So, just do cardio and relax, you know It's right... bodybuilding is bad stuff, will fuck you up!