Aerodynamics. Less air resistance results = better track times. For the athlete fuzzy legs can cost critical time over endurance speed races. The Tour de France has been won by only seconds over long distances.
Crashes, scabs and hygiene. If you scuff up your pins in a stack, long hairs breed bacteria and get stuck in the healthy goo the body makes to clean the wound. Road rash is a common and painful experience, dirt gravel and surface grit is packed with nasty stuff that often needs to be scrubbed from the wound prior to applying a protective bandage. Also bandages and band aids adhesives don’t play nice with hair, on application or removal.
Massage treatment. Remedial and deep tissue massages are a constant for the performing athlete. Hair has no place on the massage table, causing rash and discomfort.
Appearance. Pro Wrestlers shave their chests and arm cannons for appearances. Most cyclists work mentally and physically hard to build strength. So if your calves can crack walnuts, put it out there Man.