Also, if mortgage brokers are only willing to pay some portion of 14 cents CPM there, that means Facebook is creating very little value for them. Which also means that if Facebook got into the mortgage selling business, they could probably capitalize off of it in much better ways than just advertising it on their relatively worthless site.
So in essence, the only industries Facebook would benefit from entering are ones that already pay them a high CPM. But if people were paying them a high CPM, they wouldn't be in this predicament.