One contributing factor is the lovely California weather that makes it much less of a hardship to "sleep rough" than it is in most parts of the continental US.
As I said, there are many factors, weather being just one.
No, it's not. A significant fraction of California outside of the Bay Area, and some of Oregon and, IIRC, Washington also does.
What do you mean by this? Because I live in Europe too and it's rare to see a homeless person, even in the bigger cities.
Off the top of my head:
The joke is that San Diego is "72 degrees and sunny" year round. San Diego gets about 10 inches of rain annually. Fresno gets 11 inches. SFBA gets around 20. Average for the nation is 44 inches annually iirc.
If you go someplace dry that's more inland, it's typically blazing hot in summer at a minimum and deserts can have temperature swings between day and night of 30 degrees or more.
Why wouldn't the housing affordability problem in CA be directly contributing to homelessness there?
I run several homeless sites and routinely read r/homeless. A common question is "Where can I go with better weather than where I am currently?"
I spent several years homeless. I was evicted from my apartment in Georgia and walked and accepted rides to cross the country to get back to California. So I basically camped my way across the country.
I'm quite confident I know whereof I speak on this specific detail.
Though just to be clear, I am not suggesting we ship all our homeless there, though it absolutely is true that some portion of the homeless population in California traveled there from elsewhere in some fashion.