Because greater Los Angeles is the USA's (post-)WWII aerospace hub disguised as a megacity and cultural production center? All sorts of folks spent the 40s-00s (scientifically) blowing stuff up in the hills, and manufacturing the resulting products down in the basin and points south. Those businesses needed labor, which needed nearby housing, and here we are.

That's... not really a reasonable characterization of LA's urban growth patterns. To begin with, Hollywood quite clearly predates the aerospace buildout in the 40's and 50's. It was an oil production and refining hub before that, and an agricultural shipping center even before the dust bowl.

This particular neighborhood in Orange County certainly looks aerospacey, but I bet the Disney-centered service workers in Anaheim made up just as much of the population as the industrial folks.

Big cities are big for a bunch of reasons, basically. There are no simple answers at this scale.