It's even worse than that. Rain performance and rolling resistance are conflicting goals. A good rain tire will need more voids to channel the water, but adding a lot of voids increases rolling resistance.
California is a large market. Tire manufacturers probably won't make many California-specific tires. They're going to try to adjust their tires to meet the California guidelines. The most populous areas of California aren't known for demanding a lot of rain tires.
So if you live somewhere with a lot of rain, your tire choices are about to get worse to appease regulations in a state you don't live in.
It's hard to imagine median driver performance is SoCal being any worse in heavy rain than it already is.
I'm not sure the tires matter all that much if you don't alter your driving at all.
This is what I always thought. But I have Pirelli Cinturato SF3 tires now. All season. Excellent stiffness and feel. Great handling in the wet. Virtually silent up to 80-90km/h and as efficient as the Michelin summer tires and more efficient than the continental winter tires I had.
So apparently, design and compound can cater for a lot.
Unfortunately, snow performance on my front drive car is worse than my XC90 has in AWD mode with summer tires. (