Shuttle buses are well and good but few if any of the cities around LA have them. They also don't go deep into sprawling housing tracks so there's still issues of first/last mile of a commute. Where they're available they're not available in an effective volume.

I'm not saying LA should be this way, it just is this way. The suburban sprawl makes most of Southern California a shit show for public transit. Cities with effective public transit tend to have higher population densities at least more concentrated commercial areas. In SoCal (tired of typing it out) places where jobs are located are distributed all throughout the sprawl. There's no real "downtown" where all the jobs are located.

It also doesn't help that due to earthquake and fire codes it's rare to see mixed zoning in much of California. So few neighborhoods with ground floor commercial/retail and upper floor residential zoning.