I don't think Matt's wrong about the basic logic of securitization, but the problem is that not everything is a commodity, and that securitization can severely misalign the incentives between owners and managers.
Wheat (at least, mass produced industrial farming wheat) is a commodity, but individual farmers are not. Even if we accept that, yes, houses are sort of commodities (I don't), property management companies are not. And it's really difficult to imagine that appropriate contracts and be written and enforced between the owners of slices of property rental streams and those tasked with managing those properties. This was much more likely to work with mortgages than rental income streams, as homeowners have an incentive to maintain their properties, but renters don't and property managers probably don't.