The thing that limits FM pirates is the lowish power of most of their transmitters and the clear height they can get their antennas to. The FCC takes FM piracy very seriously. Successful FM pirates try to be low key and not to be noticed. If they throw up a tower in their yard/or on their apt. building, buy a high powered transmitter, and start blasting away in the middle of a large city, they're going to have Federal visitors on them quick. FM stations are very easy to DF. You've got to be in or near the area where they operate when they're on to hear most of the ones that last.
As the Dood said, the FM band in most large cities is filled cheek to jowl with stations. Even if you run low power and depend on the "capture effect" to hijack a frequency to cover your neighborhood, you're going to be messing with a station who's listeners nearby are eventually going to report you. Management is going to call the FCC, their bosses pay for them, and there goes your station. Large corporations with money and influence give a damn about pirates stepping on or close to their precious station's frequency in the AM and FM spectrum as do their poorer competitors. There's money in those airwaves and pirates take part of the audience that buy the products of the advertisers that keeps the cash rolling in.
MS-13 is pretty heavily into FM piracy in the cities of El Salvador and Honduras. As L.A. is where they formed, I've got to think their doing it there and wherever else they've got a sizable presence in the States? If you run across one of their stations, I'd strongly advise not trying to locate exactly where it's coming from. Stay safe and just listen. Those young men don't welcome strangers.