There are several reasons why it makes perfect sense, if we are trying to build general-purpose robotic assistants, to try to build humanoid robots. Now, if you want to build a robot with four legs, it's going to increase the cost of your robot. There are many unknown unknowns in this area, and so it makes sense, for the physical design of the robot, to start with a form factor we know will be able to operate effectively in an environment designed for people, and that is a humanoid robot.