Great question, when tenants switch apartments landlords have to absorb the missing weeks of rent while they are cleaning and repairing the unit. Landlords also have to find a new tenant and cover lots of other costs. Overall, property owners have to spend 20% to 45% of their cash-flows on tenants. In a way, right now long-term renters are subsidizing short-term renters.
This varies dramatically, some areas like Rittenhouse in Philadelphia has an average of 10%, while other locations have a 3% vacancy rate -- the national average in the US is typically 7%