It's been a major real estate strategy for many years to keep vacancies and increase rent on current residents. Similar to how the DeBeers company only releases a small number of diamonds every year to limit supply in order to manufacturer and artificial demand

When a complex has vacancies, the building makes less money. This difference in money is passed to the residents. Now you might be asking, why raise rent because that will make people not want to fill the vacancies? It's simple, now the property owner is making the same money as before with less tenants. Less tenants means less maintenance and less costs. If anyone fills the vacancies and pays the high rent that is just a bonus to the property managers.

With the high cost of rent, it sometimes ensures higher quality renters who make more money and are more likely to pay on time. This is not always the case, and definitely is not the primary driver in rent increases.

TL;DR: Landlords have 0 incentive to fill vacancies since it works out better for them.