Housing can't both be a good investment and be affordable
This. This. This.
http://cityobservatory.org/housing-cant-be-affordable_and_be-a-good-investment/
In other words, possibly the only thing worse than a world in which homeownership doesnt work as a wealth-building tool is a world in which it does work as a wealth-building tool.
This also means that the two stated pillars of American housing policyhomeownership as wealth-building and housing affordabilityare fundamentally at odds. Mostly, American housing policy resolves this contradiction by quietly deciding that it really doesnt care that much about affordability after all. While funds for low-income subsidized housing languish, much larger pots of money are set aside for promoting homeownership through subsidies like the mortgage interest deduction and capital gains exemption, most of which goes to upper-middle- or upper-class households.
But even markets with large amounts of affordable housing demonstrate the contradiction. Since at least the second half of the 20th century, the vast majority of actually affordable housing has been created via filtering: that is, the falling relative prices of market-rate housing as it ages, or its neighborhood loses social status, often as a result of racial changes. Low-income affordability, where it does exist, is predicated on large portions of the housing market acting as terrible investments.
And to the extent that low-income people do find a subsidized, price-fixed housing unit to live in, that means that they wont be building any wealth, even as their richer, market-housing-dwelling neighbors do, increasing wealth inequality.