While I think it's easy to demonize the investment companies (and there is plenty of evidence that in certain markets that non-resident investors drove up the price of housing considerably), basically everything I've read has said that, at it's root, the problem is just a huge undersupply of housing.
My question is: what is the real root cause of that? I know people talk about NIMBYism and regulatory capture, and that is most definitely a factor, but NIMBYs wanting to restrict additional housing has existed for many decades - why is the problem so acute now? Plus, the housing shortage is present in lots and lots of different countries with widely varying housing policies and demographics. Is it all just a consequence of the Great Recession hangover of under-building new homes?
My personal theory is that it's not so much a total broad-based underinvestment in housing, but it's that the economy has further concentrated wealth into fewer and fewer metropolises while many rural areas have hollowed out. E.g. you see depressed rural "ghost towns" in places like Japan and the US, and then you have famous examples of Italy selling old houses in villages for a euro. Thus, in many of these depressed locales, land is near free because nobody wants to live there, but prices in urban/suburban locales have gone through the roof because more people are congregating there.
That's just my pet theory though, I haven't really tried to back it up with hard data. I'm curious if anyone else has more evidence of what's driving the global house price insanity.