Strong Towns has gone over some of these reasons before, but American roads since the 1950s have been built to prioritize speed and volume over safety. The natural instinct of drivers is to drive as quickly as they can, roads must be designed to discourage that by making them feel either impossible or unsafe to speed on, unless it's a highway. The US hasn't used speed cameras on highways very often, and so enforcement has largely been reliant on physically stationing cops in specific areas. Given how spread out and car dependent much of the US is and has been for 70-ish years, and the fact that police don't have the resources to watch every car everywhere, combined with the aforementioned road designs, the environment is such that speed is encouraged by the built environment and discouraged from being enforced by lack of resources, resistance to surveillance, and possibly every cop knowing that anyone they ticket for speeding could potentially be cut off from earning a living if they have other violations.