Huh. I mean, where I come from, most sentient species are bipedal, but I think the most widely-accepted theory about that is just that bipeds are most efficient.

And there's still the elcor, they're quadrupeds, and the hanar are basically... big jellyfish. But we've got sapient life in... well, I'd say one-quarter is a generous estimate. Even with the mass relays, there's still a lot of unexplored space.