They do it because it helps support their report to the office. Without customer feedback their reports are normally just filed and won't see the light of day. At least they hope that if customer feedbacks support their reports then the company would actually do something about it.

Even if I don't get any form of compensation at least letting the airline know my dissatisfaction in the of chance that they would actually improve is good enough for me. (I won't lie though an upgrade certificate or miles would actually be good too)