Warfighters have charged into battle throughout history with fully bearded chins. Sadly, the need to survival chemical weapons attacks has overshadowed the need to keep one's chin beautiful.
Today, the most widely stated reason for requiring troops to keep a clean-shaven face is because facial hair prevents the proper sealing of a gas mask. Many studies and personnel trials have proven this true time and time again. That's right, folks. Chemical weapons are so evil that they've even killed beards.