The UK accents have developed over many years from small towns; the US accents have developed a lot quicker.
Cali - no accent? I don’t think so. The Valley accent or the Oakland accent are both quite distinctly different to one another.
Well, this is California In General. There is the “Valley” accent, which is more affectation than anything - you don’t hear the adults speaking like that.
And I didn’t say that California had NO accent, I just said it was, linguistically, the closest to unaccented American English.