It's because its used as a vehicle for bribery. See Jose Huizar the former LA city councilmember who pleaded guilty for bribery a few days ago. If its easy to get things built then no one would be paying you in paper bags of cash to get things built.

Our buildings are built to withstand earthquakes that could level other cities, because we have frequent earthquakes, and building codes were dramatically strengthened following the 1994 Northridge Quake.

A single Ritcher 5.8 quake in Virginia in 2011 caused $300 million in damage and disrupted utilities for days in 12 states. A series of earthquakes in 2019 (6.4, 5.4, and 7.1) in California caused so little damage that it's hard to find a tally. Power was knocked in some cities and restored within hours.

Yes, CA construction is more expensive. But it's also stronger and more resilient than the cheap crap in the county next to you.

There's a lot wrong but a lot of it is in your assumptions. For starters there's more than 2 skilled trades that go into a house for starters. Then by hiring them yourself either you or one of those people are going to be playing project manager to make sure they don't run out of materials which is a job in and of itself if you're not familiar with it.