You may as well ask why people play games in the most challenging mode possible. The reward isn't meant to be the real incentive behind completing the Ultimate, it's just an extra to those who have fun trying to crack such near-impossible achievements, a way of further recognizing them beyond the general personal reward of "having done this". As to whether you find it rewarding or not, it's really up to whether you enjoy playing games in this fashion; but if you're not, then you're not the target audience for this challenge. But again, fun is the main objective here, not whatever "compensation" is ultimately offered.