Why do conservatives treat Fannie and Freddie as a cause of the financial collapse rather than a symptom?

Yes, Fannie and Freddie needed to be regulated more, held bad ets and folded. But so did Merril Lynch, Bear Stearns, WaMu, AIG, etc etc. They are all symptoms of a larger problem, the fact that bankers were making bad loans not because the government had a gun to their head, but because they made a profit off of them. They resold the loans as a commodity to investors who didn't know better.