The lack of explanation about why they didn't hire you is completely normal in California. Saying nothing at all ensures they don't say anything that can be interpreted as discrimination, minimizing their legal exposure.
They have nothing to gain by explaining and plenty to lose if they say something stupid.
It's also just awkward. I've had a few cases where people pressed me for feedback, and I responded gently but truthfully. I thought the answers were somewhat innocuous (you don't have any experience with X), but people still got defensive.
I've certainly interviewed and didn't get offered a job. It stings, even when I knew I wasn't the best fit. It's natural to feel some bitterness.
They have nothing to gain by explaining and plenty to lose if they say something stupid.