I'm not sure where you got that idea. Fannie and Freddie exist for the explicit purpose of making mortgages more accessible to people who would otherwise be considered too risky to lend to (low down payments, low income, no assets, etc.). They do this by purchasing mortgages that are underwritten to lower standards and at lower interest rates than would be acceptable to private lenders, effectively shifting the risk from private lenders to the taxpayers.
Fannie and Freddie bought a tremendous amount of subprime loans. From page 454 of the Financial Crisis Inquiry Commission report[1]:
> "...on June 30, 2008, immediately prior to the onset of the financial crisis, [Fannie and Freddie] held or had guaranteed 12 million subprime and Alt-A loans. This was 37 percent of their total mortgage exposure of 32 million loans, which in turn was approximately 58 percent of the 55 million mortgages outstanding in the U.S. on that date. Fannie and Freddie, accordingly, were by far the dominant players in the U.S. mortgage market before the financial crisis and their underwriting standards largely set the standards for the rest of the mortgage financing industry."
[1] https://fcic-static.law.stanford.edu/cdn_media/fcic-reports/...