I wasn't expecting any sort of intelligent answer from Rigs... and I wasn't disappointed.

Many many thousands of mortgages come in the bundles to the FM's, or rather they come as lists with total costs from lending institutions across the country. The FM's would have to hire thousands to check item by item.
The costs would be prohibitive and the system would choke on the volume.
The government could cut down on the paper it buys, but then loans would be hard to get and business could be stifled.

Controls must be exerted on the lending institutions at the local level. Congress could do a lot by setting limits- but it is not the easiest thing to legislate. The government can, to some extent, control interest rates, and set up a climate that discourages poor loans; cyclic ups and downs could be dampened but never wholly controlled.