George Lucas said in an interview about aliens and monsters of 1950s movies:

When you have monsters from outer space or aliens, inevitably when you finally show the monster it’s a disappointment. When you try to make that real with film you have to make it exist at some point in some way that the illusion is there, that’s what you are seeing. And that’s always been a hard problem because its really technical. And from malays on its been how clever are you to trick the audience into believing something that can’t possibly exist.1