Of course not the sole reason in a strict sense. In many places, there are also tendencies of centralization that increases the pressure of housing in cities and some cities are subjected to geography that does not allow them to expand blindly. Ultimately however, the problem is one of failed politics. With sufficient planning, we could solve all of this.

Landlords and private real estate companies, often the same entities, do propagate and amplify this problem. Removing them is a step in the right direction. Short term it would crash the housing market, which is great for anyone not speculating in housing. Long term it would allow for and necessitate publicly planned housing based on actual needs instead of profitability for people that never needed the house in the first place.

Solving it is also quite easy: Raise taxation on any homes owned by someone not inhabiting it by an additional 100 % or so for each unit. Buy back some housing to be able to provide free housing for those unable to get even a subsidized home for themselves. Then treat housing as an actual need and human right, similar to food, electricity and other infrastructure.

That would be good for almost everyone and also actually good for the economy, if you give a crap about it.