Comment by AlexandrB

1 is the real reason. 2 is really down to things like a big brain and opposable thumbs. Our trunk/legs are evolved for persistence hunting and long distance walking - activities that drive approximately 0% of the economy at this point. If robots didn't have to navigate an environment built for bipeds, other configurations would be far more reliable/efficient.

For instance: a quadruped base can be statically stable in case of power loss - a biped really can't.