This simple answer is people speed because they can. Like Dog Folks said earlier, if the park enforces the rules and kicks people out, the rules get followed. It isn't any different than speeding on the highway. If you don't get stopped for 10 over the limit, many people drive 10 over. If they stopped people at 5 over, you wouldn't see the majority going 10 over.
We stayed in a private park that had usual speed limit signs posted (12 MPH I think) and when you registered, they were very clear that if you were speeding, you would be told to leave with no refunds. I didn't test it, but a neighbor I was talking to said the folks on the other side of him got the boot for speeding. He said the management gave them a warning and the next time, followed them to their site and told them to pack up and stood by while they did. I saw very few people speeding while we were there.