Can you use living creatures as a partition? What if those creatures are human beings? Can a person become part of a wall of the sukkah? What if the person doesn't know about it? Shabbat and sukkah appear to be different in this regard. Plus: If a person set off to leave the techum, and it is allowed! - and along the way, he get word that he's not needed, where does he go now? Is he allowed to return home? We can't penalize people (by not allowing them to return home) for sticking their necks out in a way that is desirable (being willing to go to begin with). The mishnah explicitly doesn't want to restrict the movement too far and inhibit people's volunteerism. Plus: A cave with 2 entrances and techum extending from each of them.