Because the Creator sets a moral standard and says, "Thou shalt not!" We don't want to hear that, but aliens provide an alternate version of the universe in which God is just a background energy which we can direct with our minds, and the "aliens" are older races that have learned how to manipulate that energy and will be glad to share their knowledge with us. The moral requirement just gets dropped by the wayside. We can do what we want, and isn't that the very essence of Satan's sin?