The argument made in science fiction literature, notably Asimov's work, is these robots exist in a wold designed for humans, and often use tools and machinery designed for humans. Making them humanoid meant one model of robot could perform a wide variety of tasks.
Of course, in his work, the Positronic brain is an incredibly expensive thing, and represents a huge chunk of the cost of the robot, so this made more sense than making every vehicle, tractor etc autonomous.