I think they are actually afraid of the notion that there are other intelligent civilizations "out there", be they in this dimension or another. Many resort to the demon explanation because they misinterpret the Bible by reading between the lines or applying weird stuff in the Bible to weird stuff in real life without a specific Biblical hook. Others seem to think they can't be there because Jesus was here, totally missing the point that God can handle the whole ball of wax without help from the peanut gallery.
Still other won't believe it because we obviously have learned all of the physics that can be learned in just 150 years; and, those things that are regularly seen can't really be there because we can't do what they do.
I attribute it to brain freeze.
I’m a Christian and believe in intelligent life somewhere out there, in the billions of galaxies which contain billions of stars, surrounded by billions of planets, the laws of probability dictate that somewhere a planet has intelligent life on it.
Like you I don’t think believing in intelligent life on other planets violates my Christian beliefs in the slightest.
Personally, in my lifetime I want to see the existence of alien life confirmed, I want to see what the government has and know what is true and what is false.
If some form of intelligent life has visited earth, clearly, they are using technology that we do not have any fundamental understanding of, which in my opinion means if they wanted to destroy us, they could at any moment, so why worry about it, welcome the intelligent life, maybe we could learn something from them, who knows.