Maybe there's a rock out there made, by complete chance, in such huge proportion of silicon that it becomes more viable, I don't give a shit. It was just an aside anyway, pick another based on the same principle if you like. Why an obligate biped? Why this size? Why not a flying creature? Why not a rotationally-symetrical monstrosity? Why not an intelligent species that physically couldn't really be engineers but happen to live on the same planet as creatures who can? Or who just get contacted by outside life that can? I'm a dipshit who mostly prefers pulp and cosmic horror (read: fantasy) science fiction, so I'm sure someone who knows more could come up with more and better prompts.