Or it could be super-low interest rates and banks giving mortgages to every stripper and then flogging them off to clueless investirs as AAA-rated securities like they did before 2008.
Or it could be because we allow foreign 'investors' to launder drug-money, blood-money ads tax-avoidance money by buying luxury apartments by the dozens and leaving them empty and unused.
Or it could be outright collusion and fraud by developers or market makers. Or a hubdred other things economists write papers about - beloved 'free market' is more than one equasion.