They are exploring the matter. For example, San Diego opened the world’s largest desalination plant less than a year ago. However, it only supplies 5% of the county’s water needs. And the water produced by this desalination plant is much more expensive than the surface water which is transported from hundreds of miles away. If voters are willing to pay a premium for desalinated water, then this may be a viable solution for California. But since so much inexpensive surface water is already coursing through the state, I think a political fight over control of that water is more likely.
It will be a zero sum fight, with cities competing against agriculture, and both of them vs environmentalists. Farmers get, by far, the biggest slice of California’s water budget. However, urban voters may want a bigger slice in the future, and consider that fight to be more acceptable than paying extra for desalinated water. Environmentalists will be hard pressed to protect the share of water which is currently dedicated to preserving rivers and the creatures supported by those.
If California was as arid as Israel, then I agree that desalination would be greatly expanded. But fresh water in California is abundant, while simultaneously limited and at times scarce. A fight over that inexpensive fresh water offers some parties the opportunity to shift the burden of water scarcity to others. I think that political fight will take place before any major reliance on desalination.