They probably are leaving money on the table, but the hassle of renting out your unused parking spot isn't really worth the money you could make. There is also the convenience on when a guest arrives with a car you can give them your parking spot (this would typically happen only a few times a year)

The landlord could make a lot of money by collecting all the unused parking spots, but that is probably either illegal (those are for the residents and can't be separated from the units) or technically legal but they fear it would be made illegal if they tried.

The real money left on the table is secondary effects: residents pay for parking they don't need/use; and that parking takes up space that could be used for something else thus making the area more dense and public transit because of the additional people in dense area can afford to be more useful.