Oh I think it's a lot of exactly what's being discussed elsewhere in this thread— the cost of consistent enforcement is just way too high, so there isn't much of it. And that suits most people just fine since they're contemptuous about road safety anyway; they want to get where they're going unmolested and have the cops focused on "real" crimes.

Road safety in the end is a commons, just like littering. No one person's action is going to make the difference, but coordinated effort is only possible when there's enough public sentiment to get a central mandate together.