Government technically isn't holding the mortgages - Fannie/Freddie/etc. buy the mortgage, repackage them into securities, and then sell the securities. The flow analogy in the article is really good - mortgages flow through a lot of intermediaries and get traded on markets. (The exception is the Fed's purchase of RMBS, which actually is a quasi-government agency holding mortgage assets. It's more like 25%, though; the Fed holds about $2.6T in RMBS, while the total RMBS market is about $10.3T.)
The socialization-of-losses aspect comes through interest rates. When wealth isn't held but is traded on markets, there's an incentive to hold interest rates artificially low, because that makes asset prices artificially high. High asset prices benefits all asset holders, so you can make people happier than they otherwise would be simply by keeping rates low.
The socialization-of-losses aspect comes through interest rates. When wealth isn't held but is traded on markets, there's an incentive to hold interest rates artificially low, because that makes asset prices artificially high. High asset prices benefits all asset holders, so you can make people happier than they otherwise would be simply by keeping rates low.