W are st of these "aliens" biped, have fingers/hands similar to humans, 2 eyes, one nose, one mouth...
Surely the odds of a human-like design being replicated on another planet (assuming there is life out there) would be unimaginably small.
The assumption of a humanoid alien is based on it evolving in similar conditions to ours. as ours is the one example of complex life forming, its the only assumption we have to go off of really.
but if its a carbon based life form, that has similar levels of whatever breathable "air", similar light levels, similar gravity, similar magnetosphere, etc etc, it would be more shocking for it to look strikingly different from us in simple terms (bipedal etc).
heavier gravity and it may have only legs, no arms, or smaller arms. less "air" it may have more than one mouth, or additional gills or something else. Darker, they may have more and bigger eyes, brighter and it may be fewer/less developed eyes, no light would mean no eyes.
its one of the fallacies of our messaging to aliens in the form of microwave. It assumes they developed along the same technological lines as us, it assumes they aren't so far ahead or behind us. It assumes they developed in the same light frequency as us, if they developed along lower frequency light waves they may see in microwave and thus may communicate along our visible light wave lengths, so our microwave verbal communications will just be weird "optical" disruptions to them.
when anything and everything is possible, and plausible, you kinda have to default back to the familiar to be able to represent them.