From an evolutionary standpoint, you want your sensors at the highest
part of your body, to look for prey or predator; you want your mouth that
high as well, to eat leaves if need be. To run from predators or after
prey you need good legs, at least two; four can be hard to get through
narrow areas, and tends to deprive one of two limbs that could otherwise
be used for tools and the evolution of the opposable thumb required for
technology. It seems a very sensible arrangement across the board for
an advanced species. (And note that of all the UFO reports we hear about,
they're always humanoid, not BEMs.)

jms