> Most road maintenance crews probably aren't sitting around

Assuming that's true, the most likely explanation is that they are working on Big Projects. Pothole maintenance is (probably) behind these projects, even though it can be done without affecting their timeline.

"Projects" (whatever that means to your municipality) are almost always contracted out. The maintenance crews maintain. Sometimes they do a little pre/post work for the big projects but mostly they maintain stuff. It's not like they're being pulled off a bridge replacement to fill a pothole.