> that they own 100%

You might be surprised to learn that many small rental properties are heavily mortgaged. Along with insurance and property taxes, the biggest cost is usually mortgage interest. Some owners even lose money for a few years until the rents rise to meet the costs so that they can break even. (These owners are playing the long game, believing that rent and property values will go up over time and eventually their investment will be a good one compared to their other investment options.)