Since when? Maybe since helmets became commonplace? About the only sport I can think of where hits to the head are a true part of the game is competitive fighting, where the object is to physically hurt your opponent. No team sport has that objective that I'm aware of, certainly not football or hockey. Aggression attracts spectators though so adding helmets is a nice way to allow for more of it without adding too many extra broken noses or head wounds (both of which cause bloody messes).