> I speculate that congress gets negative approval ratings as they are the proxy for the federal government agencies
This is not consistent with the data (over time). Congress approval used to be much higher that of agencies, with more people approving than disapproving of congress.
My take: disapproval of congress ties directly to political polarization, and the effect is attenuated for agencies as they are seen by most of the public as apolitical.
If you rep isn't "fixing" your issues with these oversight agencies, you hate the agency AND your rep.