Of course they didn't write stories of anything, let alone 'aliens' and space ships'! They thought of stories, and told them, because that is part of being human and then later the writings and pictures that there were were interpreted by people with a different and modern mind-set, who decided that it must have referred to aliens and space ships. They were wrong! :)
Sitchin, por ejemplo, interpreted a lot of stuff in way and other cuneiform experts say it ain't like that at all...."
"I never said that the ancients "weren't very bright" nor do I deny the possiblity of AAs... just don't think we should jump to conclusions..."
Their (ancient humans')brains, I understand, had the same capacity as our modern humans have, but they did not have the information we have.
Evidence for AAs would help change opinions, but any thing that is considered to be evidence is far better explained in other ways.