Or, alternatively, general-purpose robots tend to be human-shaped because the world as it already is has already been fully designed for humans. Single doors are tall because that's the size of a human. Tools are designed to be held in something like a hand, sometimes two of them. Stairs are designed to be walked on, and basically any other traversal method just falls apart.

Of course, there is also the thing where authors and artists tend to draw anything with human intelligence as humans, from robots to aliens. Maybe it's the social reason you mention, or they just unconsciously have assumed humans to be the greatest design to ever exist. But even despite this, in a human world, I expect the first true general-purpose robots to be "standing" upright, with one or several arm-like limbs.