Where did I say that?
Ultimately, the point of speed limits is supposed to be to get drivers to drive slower in areas where higher speed may be unsafe.
Cameras don't really do that very well. By the time people are 'caught' by them they're weeks past it even happening. Cops are at least marginally better at this because they don't just slow down the person they pull over, they cause other drivers who see them to slow down (its also why public announcements of traffic enforcement efforts are helpful even if it means they arent surprising anyone). But ultimately the best way to get people to drive the speed you want is to change the conditions of the road.
Unfortunately the limits aren't always set with safety being the only goal. Sometimes the speed limit is just arbitrarily set by politicians without regard to the actual ability of the roadway to handle higher speeds safely. Sometimes the speed limit is set purely with the goal of generating revenue.