Our imagination cannot imagine something from nothing, so the aliens from the movies are constricted by the possibility of our mind. We cannot think of something that is too abstract and that's why it's much easier for us to imagine an intelligent being as a humanoid.
We can easier portray an intelligent being looking like us, or better said, like what our subconscious view of what we think we'll became. Humans are loosing their dentition, so aliens have a very small mouth. We think of them as smarter, therefore, a larger head for a larger brain. We assume they don't use brute force, but their brain, so their body doesn't need to have a lot of muscles. Our most priced sense is sight, while, most animals we know and live close by (for example dogs) have a better sense of smell than us, so we create aliens with very big eyes and no or very little noses.
Why do they all look the same? When someone creates a formula that spells success, why change it?