Gangs and all that come with them are the natural immediate response to societal breakdown. It is evident in any place where civilization breaks down.

Notice that in order to thrive people are suddenly given to simple tasks and vocations.

This poverty can come suddenly into an area (like Portland, Or, California, etc) and suddenly the way in which life works has dramatically changed.

In major cities the gangs are uncontrollable once civilization breaks down and raw survival kicks in.