Inelastic supply. If the price of houses goes up 20% it isn't going to convince many (or any) people to sell their houses and move back into apartments. If there is land and zoning laws aren't too onerous, it might spur more new construction but new construction takes longer to hit the market, and in many of the biggest and most expensive cities isn't a significant contributor to housing supply at all. So if more people want to buy houses than there are houses for sale, prices have to keep rising until enough of the people who want to buy houses are priced out of the market. At that price point supply and demand equalize.
But it has always been the case that more people want to live in single family homes than the total number of such homes. And it has always been the case that the limiting factor isn't how many people want to buy homes, but how many people can afford to buy them at the current price for houses. So I don't think the problem right now is too many people wanting to buy homes. It is that, through a combination of very low interest rates and a huge increase in savings rates over the last year, many more people can afford to pay much more to buy a house. So it takes a much higher prices than in the past before enough people cannot afford a house for supply and demand to equalize.
TL;DR My view is that high housing prices aren't driven by too many people who want to buy homes, but by people having much more money to spend on buying a house than in the past.
