Owners typically manage only smaller properties. E.g. an in-law suite or basement apartment in their home, a duplex, or perhaps a smaller building with, most likely, less than 10 units. Larger complexes are commonly owned by corporations or groups of investors, not a single individual, and hiring professional property management firms is required. The management company has a fiduciary duty to the ownership, and therefore has stricter requirements for financial data, credit scores, etc.
You rarely, if ever, can find apartments in small buildings or private homes listed on websites such as apartments.com, etc. It costs too much. You have to look at ads in local newspapers, or websites such as craigslist (beware the scams on this!) or similar free places to list.