As well as holding onto housing stock to maintain a local monopoly.

It isn't as big a deal when you know any potential renter will be renting from you regardless of it is this unit or that unit. They are still forced to be your customer and you don't have to compete with lower prices from another landlord.

Fair enough, it does happen. Relevant background: https://nationalmortgageprofessional.com/news/investors-hold-most-empty-houses

The states with the highest vacancy rates in investor-owned properties were:

Indiana (7.2%); 
Illinois (6.1%); 
Oklahoma (5.9%); 
Alabama (5.9%); and 
Ohio (5.8%). 

States with the lowest vacancy rates were:

New Hampshire (0.9%); 
Vermont (1%); 
Idaho (1.2%); 
Utah (1.5%); and 
North Dakota (1.5%).

I'd argue my sentiment remains, investors for the most part rent out property, they don't typically hold it vacant. When they do it's more likely vacancies are transitory (trying to find a renter) than long term and intentionally kept vacant.