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The alien xenomorph is bipedal for the same reason most BEMs are bipedal: it makes it easier for the costume and casting departments.
With CG, our aliens will be more imaginatively designed.
What would a real alien looklike? There's no way to answer that, of course, for obvious reasons. But my guess is it will be totally strange and unexpected, although parallel evolution will provide it with several creepily familiar structures.
Consider, a critter that looks like a cross between a tarantula and a jellyfish, with eerily human-looking feet.