How come some police cars are not equipped with lightbars?

Some police cars are not made obvious by placing a lightbar on the roof. It is easy to spot a regular patrol car in traffic, because the lightbar stands out above the roofline of most passenger cars. If police wish to be able to observe, tail, or otherwise operate without their presence being known, a lightbar is a hinderance. These are not undercover cars, being operated by detectives, but patrol cars operated by line officers, generally speaking.