Law enforcement is not equally distributed so criminals also avoid certain areas to commit crimes in because they know they will get caught or killed by police if they do. But that also means they know certain low population density dilapidated areas that cops (and most everyone else) avoid, where they can easily get away with crimes, because not only are there no cops anywhere near, but the few people in those areas know not to talk to the police and help them catch you because they know they have basically zero protections living in that area from retribution.

Detroit is a very expansive place, and there is basically zero reason to go into the run down dangerous areas unless you are also dirt poor or involved in criminal enterprise.