Police officer here. The reason that police aren't "professionals" is becuase we aren't independently licensed. Doctors and lawyers own their licenses and can independently practice anywhere they want with thier only real oversight being the boards that certified them. Officers, on the other hand, are POST certified but can only legally work as LEO's when commissioned by a governmental agency. There are multiple layers of people who oversee my work, and I could never start an independent law enforcement agency.

Also, on the liability area, it makes sense to penalize the government when I am acting within the scope of my duties and training. I can have personal liability if I don't act with reasonable prudence, however, and my department can and has declined to defend officers in law suits.