None of those places have Prop 13 or California's environmental rules, so why can't they build appropriate density?

They are. But it will be built in the context of a market economy. If you want someone to pre-build a city for a million people, try China, which still has "ghost cities".

Presumably it takes time and a continual influx is going to leave the demand constantly ahead of the catching up supply.

Skilled construction workers are a limited resource.

Skilled construction workers that aren't yet half-disabled, you mean. They have a shelf life, not unlike those of professional football players, stuntmen, longshoremen, firefighters, soldiers, pre-Industrial-Revolution farmers, etc.

"Back-breaking" isn't just a figure-of-speech; modern medicine isn't (yet?) advanced enough that a rational actor should ever choose construction work over something else; and there are just enough desperate people around that I doubt anyone is willing to pay the true cost of construction-work to a not-as-desperate would-be construction-worker.

The limited resource here, then, is actually: desperate people who are still smart enough to do construction work.

I can't wait until construction becomes a non-human-powered activity - even if it still requires a human to control the machinery.

Yeah, I don't buy this. The problem is more about the cyclical nature of the construction industry and the trend to push people to get a college degree and away from the trades which pay pretty well it's not as back breaking as you are making it out to be.

The cyclical nature would be overcome in the relevant areas if, as stated elsewhere in this thread, this exodus from the Bay Area is a slow and steady trickle.

That trend is so powerful precisely because lots of trade-working parents don't want their kids to suffer the kinds of medical bills, chronic pains (with potential resulting opioid addictions) and stunted healthspans, that they do.

Granted, construction is rather especially bad among the trades in this manner: things like welding are not quite as physically-demanding.

Most housing construction is done largely by unskilled / semi-skilled workers with a few skilled tradesmen to supervise and do the complicated parts.

Housing supply isn't very elastic.

I think complaining about housing prices is in vogue because saying "we just hate your politics" is very much not in vogue.