Psychology — our minds are thinking about what we do next, so the present is regularly seen as an impediment to getting to the future — our destination. If we speed, we are able to do things we want to do next, sooner. After which we will rush to do the thing after that.

Physics — the roads are built according to how curvy the land underneath them is, and according to how much reaction time you need to respond to changes on the road — for example due to weather. Most people speeding are decreasing their calculated and often legislated margin of safety.

Economics — automatic enforcement is not necessarily cheaper — and when it is deployed, people just learn where it’s deployed and stop breaking the law in that one place. In many places traffic fines pay for public services, so you don’t actually want to stop the behavior outright, you just want to catch the people in the act.

Economics — safety isn’t the only constraint on speed — travel time is important. People tend to do more ecologically important things when they arrive compared to when they travel. Significantly slowing everyone down would have a measurable impact.