The problem with the 'good cop' is that they're either too incompetent to know who the 'bad cops' are, or they know and they do nothing about them, or there are too few of them to do anything about the overwhelming number of 'bad cops' (the real answer). But in any of these cases there obviously aren't 'a lot' of good cops.

I do feel for the few that exist, but police abuses have been institutionalized so even a 'good cop' is working against bad policies and will eventually fall in line with those policies or not be a cop for long.