Because not only does the camera operate 24 hours a day, without vacation or potty breaks, but the $6,000 a month covers all of the camera systems, not just one. How many cops will $72,000 a year hire? Keep in mind that the cost of a cop is not just salary, but also benefits, insurance, equipment, and so forth.
Also keep in mind that while the cop is chasing down one driver, writing the ticket, and getting back 'on station' six more have blown by Scott free. The camera would have nailed all seven.
It isn't 'cost per year' that matters, but 'cost per conviction'.
I should have added a down side to the "savings" of not hiring the cop over the cost of cameras: When a cop stops the vehicle, he can often write or arrest for other things; a camera can't.
The cameras don't differentiate between a simple lack of attention, and a drunk; both are equally mailed a simple red light ticket.
The camera won't see the drugs or other contraband on the seat or floorboards.
The camera will ID the driver, but not the wanted perp in the back seat. For that matter, the citation may be mailed, but the driver with warrents out on him won't be hauled in by the camera.