More helpful responses can accidentally open you up to discrimination suits, and given the option of (A) bulletproofing every helpful response to make sure it won't come back to bite you and (B) just giving a generic and safe response every time, you can see why they'd choose the latter.
Even those who actually want feedback are often unprepared to hear it. Thinking back, some of the biggest things I struggled with were things that were invisible to me. If I try to imagine explaining some thing to my younger self, my options are a) explain it politely but in a way that I wouldn't get, or b) being so blunt that I get upset or defensive. E.g., what I once saw as being passionate about getting at the truth is something I now see as me acting like an arrogant, argumentative, know-it-all jerk.
Plus, feedback is much harder when it isn't interactive. When I do give feedback, it's always at the interviewee's request, it's always right after the interview, and it's always in person. Then it's pretty easy to gauge their emotional state, give them the feedback that's useful to them, and refer to specific examples that are fresh in our minds. Doing that a couple weeks later in print would be a nightmare.
[1] http://whatever.scalzi.com/2007/01/23/what-to-know-before-yo...