It has been difficult, time consuming, and expensive to depict aliens as other than people in costumes. Aliens in scifi novels of the even of the past were often much more non-human appearing than the on screen counterparts.

Many stories featuring aliens have elements of morality plays or other abstractions of human interactions which are ported to a scifi setting for the purposes of abstracting them.

Some stories with humanoid aliens are wholly unconcerned in exploring the aliens as something truly alien, so aliens are simply different people more or less.