"In the United States, the most common securitzation trusts are Fannie Mae and Freddie Mac, U.S. government-sponsored enterprises."
"In 1938, the government also created the government-sponsored corporation Federal National Mortgage Association (FNMA), colloquially known as Fannie Mae, to create a liquid secondary market in these mortgages and thereby free the loan originators to originate more loans,"
http://en.wikipedia....e-backed_security
Buying mortgages is what they do, expressly to provide liquidity to the market.