A home is not shelter merely because it exists. It becomes shelter when someone is allowed through the door.
When expected asset gains outweigh the cost of keeping a habitable dwelling empty, vacancy becomes privately rational. The gain stays private. A person outside carries the weather. Everyone else carries the human cost.
Not every vacant unit is safe, usable, suitably located, or deliberately withheld; turnover, repairs, disputes and seasonal use are real, and homelessness has more than one cause.
But rules that reward long-term non-use can be changed. Vacancy measures and incentives for occupancy can return some homes to use, though they cannot replace affordable housing, income, care or support.
The shelter has already been built. What is withheld at the locked door is permission.