Because they don't have to.Fannie Mae and Freddie Mac are Government Sponsored Enterprises (GSEs). As such, there are transparency requirements that they must follow. Similarly, the FHA, as a government agency, is required to disclose. I believe the VA is also open about their underwriting.But private lenders, unaffiliated with the government, may see their underwriting process as a competitive advantage or trade secret. And thus choose not to be transparent.