The point of regulation isn't for the state to turn a profit. In fact, I'd go as far as to say that regulations that drive a monetary profit for the state are generally bad because they create a perverse incentive. For example, municipal governments adversely affect traffic flow by lowering speed limits because those lower speed limits generate more ticket revenue.
I don't know about the US because the US is weird, but:
* at 30kmph the rate of fatalities in case of a car hitting a pedestrian is basically 0%, at 50kmph I think it's 5% or more
* at 30kmph collisions are much easier to avoid due to the increased reaction time and the decreased braking distance (I don't remember the exact numbers)
* at 30kmph you can hold a conversation at normal speech levels next to a moderately busy road, at 50kmph you will have to shout (and not even notice it due to the high ambient noise)
Etc.
You could create a private right of action for this, but that is its own bag of worms.
Seems empirically false given e.g. the immense investment we make in counter-drug enforcement.