It’s the ongoing maintenance, just like how calibrating cars to exact speeds is expensive, so too are radar and laser guns. So the burden is on the radar/laser guns because there are fewer of them. And the calibration cannot be performed on site, as the unit your testing against also has to be calibrated to a much higher degree and you’re not going to throw it in the back of a truck and drive it around risking damage. And in the event of someone disputing, it’s up to the state to produce proof of calibration rather than the individual proving it was not. It’s an expensive process, as anyone who uses specialized tools (not limited to law enforcement) would tell you.

Now, it’s possible that the calibrations aren’t done, and people don’t question them and get tickets. But you want the ones that don’t constantly make mistakes. 20ish years ago DC put up cameras and it was sending tickets across the entire US because it wasn’t reading plates correctly. They had to hire more people to cross reference any matches with the make and model of the car just to double check, and that was a lot of eyes that needed paying. Even though a lot of it is automated today, you’re still paying a premium over putting a camera on a stick.