As an LA resident engineer myself, my theory (backed by an extensive amount of no facts whatsoever), is that engineers are more likely to "end up" in LA than to deliberately move there.

Almost every coworker I have had is in LA because they needed to move closer to family due to illness, they moved there because their girlfriend/wife is in some grad school program at UCLA, their girlfriend/wife is trying the acting/performing thing for awhile, etc. Engineers in situations like those don't have much flexibility in taking much under market rates and basic perks (namely health insurance).

Also, as Paul Graham said in one of his essays, Los Angeles' identity is "you should be famous," and unfortunately, software engineering isn't known for breeding famous people outside of our TechCrunch echo chamber. In the Bay Area, founding a successful startup makes you a rock star. In Los Angeles, just about the only thing you can do to be a rock star is to be a rock star.