Simplicity sake. SPECULATION AHEAD animating something with multiple legs is more-a-less a more manageable thing now-a-days but I assume it's still a bit...uh, tedious. It can even be buggy on very uneven terrain. To the point that multiple legs can glitch and spazz about merely because the mechanics in the animation cannot decide where to put the leg. Most of the times I see this though it does result, but breaks the "immersion" of things. Another thing could be, in terms of any type of body form, designing technology for said species. You'd have to think into their evolution, and how they'd build things to accommodate them. While this merely takes time and imagination you then run into a problem. Humanoid characters, the which that you'll be playing, cannot utilize these vehicles. I think that's why Bungie decides to not use non-humanoids, or at least not a notable amount of them. The ability to run about and use the enemies toys against them in an ever so gleeful fashion is rendered moot and void in the face of machinery that, for you to operate, would require you to spawn limbs or be a contortionist to fit in.