I think it's in large part because Columbo isn't really trying to portray a "real" police officer, but instead a pleasant fantasy. Shows that try to emulate what's real are also usually fantasies, but they don't portray it like that, instead trying to portray abuses as "good" or "necessary", and dealing with serious topics only on the surface level.
Columbo doesn't wear the uniform most cops do, he doesn't really do "beat cop" type activities, he's the kind of cop most people will never interact with. He also has a very distinct and recognizable persona and style. If you take a look at the worse cop shows, the police officers tend to be very similar, less distinct.
Also, I think that "The Wire" is pretty good, despite being a cop show. It tries to be real, but it does a much better job than most cop shows. It's not all black and white, and doesn't hide a lot of the issues the way other shows do.