When I was a competetive swimmer we were told to shave because (a) it was as good a feeling as swimmimg naked therefor it gave a real psychological boost for the really important event.
Likewise shaved legs make you feel good.
I'm not convinced that eirher sport would actually benefit aerodynamically shaved or not.

Real reason is the crash thing. believe me haveing dressings changed and wounds cleaned over hairyness aint no fun (Not that's fun if you're shaved - it's just less painful!