And in reality they do, O-linemen hold basically on every play. But if a blatant hold is what opens up the running lane for a big play or prevents a sack it has to be called.
The sidelines hit though is totally different, if the guy is going to catch the ball in the middle of the field you can flatten him to try preventing him from catching the ball, that should work on the sidelines too. Plus it takes a judgment call away from the refs, either the guy got both feet in with possession or he did not.