Mostly in these situations the cops know darn well what is going on, and for what ever reason do nothing about it; sometimes it is strategy, sometimes they just do not want to exert the extra effort from a priority perspective. Or in cases where multiple crimes are committed only charge one of the parties[remember Rodney King].
We had one of these near us where a couple inherited a house an dealt drugs out of it, it looked like a drive through at times…and nothing was done.. the house gradually ran down, and luckily they lost it. The next guy was not much better. When it was vacant strangely enough. Fire broke out and the response time had to be the slowest we had ever seen, then once there, the water only flowed when the houses near by became endangered….the fire department ensured that our problem was not going to return and solved what the police did not. I t then be came a rather nice looking grassy lot. The arson investigator gave the guy who reported the fire a bit of a hard time, but john couldn’t walk fat enough to save his life, so was off the hook.