As someone who has driven on southern California freeways many hundreds of times, I speak as the voice of experience. (For those of you who don't know, an experience is a former perience.) Most drivers on our congested freeways don't bother to signal before they change lanes. The reason? Other drivers are very protective of their "space" on the road and seldom let another driver get in front of them. Sometimes when they see another driver's turn signal, they'll even speed up (if possible). We've learned to wait until a gap opens up between vehicles and then quickly move into that lane without signaling.
You've heard of "defensive driving"? In Los Angeles we have offensive driving!
This is not only common to SoCal. It's an everywhere thing. In every state. In the city. In the country. Going into Six Flags. Going into construction.