The two largest demographics (Millennials AND Boomers) are in houses right now. Boomers are living longer and better and want to stay in single family homes. Millenials are having families later so are just now buying homes but with greater buying power.
I think that’s it. Really, the explanation is simple. It goes back to supply and demand. Tons of people need houses right now and supply can’t keep up. Kind of morbid, but I think it turns over the next decade as Boomers “no longer need” their homes.