There's also stuff that breaks unexpectedly that you're on the hook for repairing immediately, at whatever the cost. It can sometimes be difficult to fill vacancies, and in many instances those "binding contracts" aren't worth peanuts. If a tennant leaves or destroys the apartment, you could either go after them in court (expensive, time consuming, and you aren't likely to recoup very much very easily, and you certainly can't compel them to continue to pay you if they're broke), or live with keeping their security deposit (again, peanuts).