Ever since the rules were changed to allow "push-and-dance" blocking it was only a matter of time before the situation deteriorated to the point it is today.
If I had to guess, I'd say it is probably due to TV ratings. Think of the NBA. If they called walks, Michael Jordan would have turned the ball over every other play. Why don't they? Because the fans like offense and they like high flying play around the basket. Hence, no calls for walks on people who can provide the viewing experience.
Same difference with college football. Fans like to see offense, especially passing; it's "exciting" and all the rest of the pundit hyperboles. So, slowly, over time, a "going rate" for holding gets established and the refs, without any orders, begin to conform to it. It's sort of like spanking kids in schools; it's against the law and school policy everywhere, but it happens. It's usually because the word gets passed around that if you lick a kid nothing will happen unless you get caught at it. (The social science types call this "informal institutionalization".)
And, of course, you can just get real bad refs, like that band of clowns who called the Puke game.
