Besides being (I would guess) slightly more crime-friendly than other places, I'll bet part of the problem is the expense of catalytic converters in CA. They are probably worth a lot more as spares than as raw materials.
It seems to me that CARB forces people to buy OEM parts, or at least some kind of on-a-list part for $$$.
I wonder if shops will still install used (stolen) units regardless? Or maybe they are just being shipped out of state or broken down for raw materials.
C'est la vie. You're not supposed to steal cats either.
The one thing that's fairly hard to get around is the biannual inspection.
In days of yore, a box of donuts could get someone to hot pipe your car. Now, not so much. It was kind of a necessity if you owned a car that wouldn't pass even in absolutely pristine new condition.
It seems to me that CARB forces people to buy OEM parts, or at least some kind of on-a-list part for $$$.