REITs have been doing this for a long time. It accelerated after 2008 because of the housing collapse which was triggered by Banks creating to much money rather than the Fed.
Fannie and Freddie bought up a lot of subprime mortgages.
Red herring. Most origination of subprime started with private market actors. GSEs were a fraction of the market; very late to the game and never held anywhere near what investment banks did.