I'd imagine it's a lot like going to NYC and just randomly walking into a place for a slice of pizza or a bagel. You might luck out and walk into a good spot but chances are it's gonna be disappointing unless you know someone who can guide you to the good stuff.
I think it's like that everywhere. Go to any BBQ city and there's bad/mediocre places mixed in with the good/great ones. Locals are just reluctant to admit that they've got some of the worst places mixed in with the best, because when they talk about BBQ they sure as shit aint talking about the places they won't go.
It's almost like when you have BBQ, Pizza, Greek, Italian or whatever on every corner, it's because everyone and their brother opens one of those places, including the fuckers that are terrible at it.
It was like that when I lived in Buffalo. Lot's of pizza places everywhere. Had some of the best pizza there, but also some of the absolutely worst garbage pizza ever.
I didn't spend enough time in LA to form an opinion, but to hear people from LA talk, they have the best food of every type, anywhere. And I'm sure if you spend the time to suss out all the hidden gems you can find a bunch of amazing places. But you can't tell me that LA doesn't have tons of garbage-tier places, either. And LA strikes me as one of those places (like Texas, NYC, Chicago, etc) where locals will go on at great length about how their food is better than everywhere else, w/o exception, to the point that you start thinking of ways to bail on the conversation.