There are people who own vacant houses that they have zero interest in renting, and are not looking to sell. This could be a vacation home for a billionaire that they seldom get around to visiting, but want to have ready for them, and just have staff check in periodically. Renting it when not in use may be uninteresting (have to lock up their belongings, arrange some form of management company, have insurance concerns (if somebody realizes they are renting from a billionaire, they might get "hurt", hoping for a big quick payday) etc.)

Similarly there are vacant houses own by speculators that have zero interest in renting out the unit, and are not currently looking to sell, since the market isn't right yet.

This is among many other scenarios where vacant does not mean available to rent or purchase (well at least not at anything close to market rates. Some of these people may reconsider if offered way above market rates, but that generally only happens if a property is uniquely valuable to just that one buyer.)