Why is the address of many cities in LA labeled Los Angeles, instead of their city name?

It's pretty common for the postal address to be the major city rather than a smaller city in a metropolitan area. This is because the sorting center is supplied by the major city.

Because you might have thought you were in Culver City, but you were in the City of Los Angeles.  If you were in Culver City, the address would have been Culver City. 

The Palms, Westwood, Mar Vista, Cheviot Hills, Baldwin Hills, and everything around Culver City is the part of incorported City of Los Angeles not part of another incorporated city. 

There are many small cities and unincorporated areas that are completely surrounded by the City of Los Angeles.  Zip code boundaries don't always match up with city limits, so those areas are served by post offices in LA.

Some cities are no longer incorporated as separate cities. They become part of LA, so the exact address reflects this. Areas like cheviot hills and parts of Beverly Hills are incorporated into LA.