You have to pay for upkeep...electricity, gas, landscaping, local taxes, etc while you own the home. It also ties up capital for future purchases. They don't want to do that indefinitely as it would eat all the profit. They've probably done the calculation, realized that losing 1% (or whatever) is better than keeping the house on the books and making a small profit on the sale, but being negative after all the money they have to spend on ownership.