Why can't companies provide feedback iff the candidate agrees that the feedback is subject to NDA? Interview candidates already sign NDAs for the coding questio

Why can't companies provide feedback iff the candidate agrees that the feedback is subject to NDA? Interview candidates already sign NDAs for the coding questions asked. Granted those NDAs are always ignored (based on coding interview sites) but the NDA would at least provide the "protection" that these companies want. Then the companies benefit from having candidates actually improving and potentially coming back hirable.