Possibly because head vs head nobody leads in with a head challenge carelessly ,recklessly and without expecting to connect with the soft ball .You still haven't given any good reason why. Head injuries are treating more seriously, yet head challenges aren't. A total contradiction.
The bigger difference with foot and head is neither player "leave their head in" "follow through needlessly" that can happen with the foot .
I'm not trying to compare deliberate fouls but those that are considered accidental injuries caused.
Rat hater than head challenges I'd rather you compare with goalkeepers leading with raised knees in the6yd box - these are never called fouls