The persistence of government mortgage subsidies is explained by a rent-seeking story rooted in 1980s Wall Street (as described in Michael Lewis's Liar's Poker): the huge U.S. mortgage debt market became profitable to securitize, but securitization only makes sense if the embedded options are important—you can carve 30-year mortgages into tranches for profit but not five-year mortgages—so Wall Street worked the political process to establish and entrench mortgage securitization.