Hundreds of thousands of people live in high density areas of Los Angeles with densities far exceeding San Francisco's most dense areas. Also people live in LA for the same reason they live in other big cities, because it's incredibly diverse and vibrant. Traffic may be bad, but most people aren't in transit for that long anyways since the city is still pretty dense, and you can get everything you need within your neighborhood (where many neighborhoods are totally walkable).

but (hundreds of thousands[living in high density LA]) + (hundereds of thousands [living in high density SF]) + 8 million (living in NYC) =/= 10% * 350 million.

In fact, if we generously estimate that 10 million people live in high density metro areas that don't rely on cars we are looking at less than 3% of the US population. I think that is something to sneeze at.

100% of the my friends and family who live in New York would live nowhere else in the country in spite of the extreme costs of living in Manhattan, when they could have equal incomes elsewhere.

I know it's hard for you to believe, but some people like living in high density areas.

I understand that. But at the same time I know plenty of people who live in SF who would love to move to a more rural area, but they or their spouse can't find work in their field outside of metro areas and SF is close to their rural roots. Some people don't like living in high density areas, but find themselves their out of circumstance.