The article you linked is misleading. Houses are vacant for various reasons. Some are temporarily vacant:
some are undergoing renovations
some are between tenants
some are for sale
Some are more permanently vacant because they’re in such a state of disrepair that they can’t be lived in.
Rental property owners rent out properties, which keeps people housed and off the streets. However there’s been a lack of housing development over the past decade in the United States which leads to a housing shortage.