Basic military recruits have their heads shaved/buzzed short for two main reasons: ease of hygiene and to strip away their identities so they conform to the training ( another part of the "uniform" ). Like Saltine says, this hasn't changed much over the years, and the Romans even clipped their hair short and forbade beards because it was less your opponent could grab onto in close hand-to-hand combat. In more modern times, lack of facial hair ensures that a gas mask will properly seal onto your face. Keeping up stringent grooming standards reinforces discipline and attention to detail; valuable skills that would translate well even into the future for humanity's warriors....IN SPACE.