Do police not arrest people for this?

Depends on the city. Some cities are notorious for using speed traps to increase revenue.

From Texas to Ohio, municipalities are using law enforcement to counteract declining tax bases through the aggressive enforcement of fineable offenses such as speeding. A 2019 report estimated that nearly 600 jurisdictions nationwide generate at least 10% of their general fund revenue through fines and forfeitures.

Other municipalities have enacted their own policing-for-profit programs. In Brookside, Alabama, the town of about 1,200 residents saw its revenue increase more than 640% in only two years, according to AL.com, after police began an aggressive traffic stop and ticket-writing campaign. Fines and forfeitures made up almost half of the town's budget.