So do Dallas, Atlanta, Tuscon, Indianapolis, and a bunch of other extremely red cities which have an excess of law enforcement. It turns out that the amount of law enforcement isn't related to the presence of gangs.

What prevents them from offering a "service of getting rid from problem tenants"?

Because outside of Hollywood movies, life doesn't actually work that way. If problem tenants suddenly started disappearing, it would be pretty easy to follow the clues back to the gang and the landlord, and the punishment for both would far exceed the economic benefits to the landlord or the gang. Law enforcement many turn a blind eye to drug sales when they have to triage prosecutions due to limited prison capacity, but they aren't going to turn a blind eye to murder.