They built it somewhere someone lived and owned the land, they do not built a town in England of high density in a field that no one has occupied for generations, connected to new roads that didnt exist before.

Yes as we add density we do add some items like extra schools, sure, as the population is growing.

No one is going to build a town of 100,000 off a new non-existent highway they also built somewhere in CA. It's going to go somewhere that has some density already (and has that density usually as it is near something desirable).

They also dont built is near where THE sfhs are (as in all), they built it near SOME of it. Thus the NIMBY desire for it to fall somewhere else. We cant make ALL the sprawl dense, we don't have the people for it. If the Bay Area counties had pop density of urban areas of East Bay across all of it, it would fit every CA resident with none left to populate LA, San Diego, Sacramento etc (not a recommendation, just a note that density can't be everywhere).

From the outside, it seems like Canary Warf was where most of the high density building in London was done. That wasn't nearby lots of low rise single family homes, was it?

Certainly, some people are moving to the Bay Area, but the rate of home building well exceeds the population growth rate of California (or England) which are occasionally negative. It's mostly a question of new people wanting to move to particular places they like. But ask, why do they like them, and how was that created? Also ask, what fraction of the residential homes are occupied. In California (especially in desirable areas), the answer to that is likely <90%. Nobody (especially not developers) want to raise taxes on empty houses/apartment/condos/retail, but maybe it would fill up if they did (and lower prices for people who want to rent/move in.