Doesnt seem to be many SBM heads that are "too slow" to begin with either.
I think people worry themselves with port velocity to rationalize not going to greater lengths with their porting. Over-porting seems mostly to be a worry when people focus on the wrong areas and wind up with wide and narrow spots along the port or just blow out areas that are easy to reach.
Or they just ruin really important spots thinking they're improving things...
Also because people are cheap and think offset rockers are too complicated.