Pro racers are going for speed and not efficient use of energy of which they have plenty to spare. Watch film of the riders going up a grade and seldom will you see them out of the saddle unless breaking away from the group near the end of the stage. The best mountain riders will be in the saddle as they do not waste energy supporting themselves and can be more balanced on the bike.
I can stand up to pedal up a hill but I know that I am expending much more energy than if I am seated on the saddle and using a lower gear. The lower gear also allows for a higher cadence, though not nearly as high as pro racers commonly use.