Most people will never interact with a cop on duty outside of a speeding ticket or some other mundane encounter. A major chuck of what many people think about police comes from TV and movies.

It's impossible to overstate the influence of Dragnet (the OG police procedural from the early 50s) alone on the widely held idea that police are mostly heroic and good. Police procedurals are still extremely popular, they overwhelmingly portray law enforcement in an extremely idealized way.

There are exceptions (The Wire, The Shield), but they are noteworty in that police are not heroes.