Agree, but to be fair, the refs can't watch 5 OLmen, a TE, and a RB at the same time. Some holds will get missed.
I'm not so worried about the ones that get missed honestly. I'm worried about the blatant ones that are clearly seen and not called. There are a lot of them.
I realize the refs do not want to slow down the game, and no call is more annoying to a casual fan than a holding call, because to them it happens away from the action. Unless an OL tackles a DE with a clear line to the QB, the casual fan often prefers the game to just go along. So, the NFL understands that and the refs understand that. But, it leads to the refs having a significant discretion on whose offense is effective.
Big teams in big markets with big fanbases, like the New England Patriots or Dallas Cowboys, always seem to get the calls. Smaller market teams with big traditions and a currently large fan base, like Pittsburgh and Green Bay, get the benefit of the doubt often as well. Media darling QB's like Andrew Luck rarely get their offenses derailed for holding either. And home teams are usually cut some slack as well. Fortunately, we're the home team half the time (when we're not the London "home" team), so we've seen our share of it as well. But, more often than not, it works against the Dolphins.
IMHO, the refs could call and enforce holding much quicker. Have one guy physically move the ball, while the clock runs and the main official voices it to the crowd. It barely slows down the game at all. But how it is done now, slows it down tremendously. First, the flag is thrown and the whistle blown, then the officials confer, then the official announces the call, then he goes and picks up the ball, walks off the enforced 10 yards, then restarts the clock. Half of that could be done simultaneously, with far less game delay, more time for sponsors spots (which the NFL wants), and less annoyance to the fan. Just have the flag throwing ref mic or hand signal the call to the other refs, then have the main guy voice it as another ref walks it off. Maybe it would be called more if it were less disruptive?