You risk negative returns on the property, or losses compared to your opportunity cost. If you had $1 million in 2007, it would have been more profitable to in

You risk negative returns on the property, or losses compared to your opportunity cost. If you had $1 million in 2007, it would have been more profitable to invest it in Google than to buy a two houses in Mt View and rent them out. That is the risk the landlord is assuming. There are examples where you have a negative return as well.