I doubt most people think that. I grew up on the central coast and never heard that and large areas in the oil fields of the central valley will make you think you're in west Texas. Yah, it's just Hollywood movies and a tiny subculture of the states population.
I have homes near Santa Cruz and San Diego beaches and I've yet to come across anyone that talks like beach bums. However, when I travel anywhere near the Appalachians, Hollywood seems to got that part somewhat right.
I live in Los Angeles County and lived in San Diego for over 10 years. I was a teen when the song Valley Girl came out. I went to high school in the same district as the high school that was the basis for Fast Times art Ridgemont High (Cameron Crowe was undercover as a student at a San Diego High School when he wrote the book that was the basis for the movie). These two items alone are a huge part of perpetuating the myth that Southern Californians speak in that manner. I hung out with the "surfers" and "stoners" in high school. I am 48 yo and have never heard anyone speak like in that manner.