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Why are residential investment properties owned by non-professional investors and not large corporations?
As other answers have pointed out, professional real estate investors do own residential investment properties. However, small residential units typically are not owned by professional real estate investors as your experience confirms. This has a fairly natural cause. The size of the investment opportunity is insufficient to warrant the proper research/due diligence to which a large investment firm would have to commit if it wanted to properly assess the potential of a property. For a small real estate fund managing, say, $50 MM, it would take 100 properties at a $500K valuation in order to fully invest the funds. This number grows quickly as we decrease the average valuation to reflect even smaller individual units. Analogously, it is unlikely that you will find large institutional investors buying stocks with market caps of $20 MM. They simply cannot invest a large enough portion of total AUM to make the diligence make economic sense.  As such, institutional real estate money tends to find its way into large multi-family units that provide a more convenient purchase size for a fund.