Humanoid Aliens

Why do UFO proponents and other supporters always seem to depict their aliens as humanoid? Why are they insistent that these alleged life-forms follow the human body-plan? This goes against our understanding of science and all current biological models of how life evolved on earth, let alone other worlds. Even if the entire process of evolution on earth were re-simulated with all of the original parameters of biological genesis in place, we still wouldn't come out looking the same. So why would a visitor from an alien world look like us?

It's a near guarantee that they wouldn't.