Why are so many people willing to believe in the existence of extra terrestrial intelligent life [aka aliens]?

I believe it is a simple answer. People don't tend to acknowledge what to do not understand. In the ancient Hebrew Bible, (the one apparently where our Bible is translated from), the word Heaven means sky. Therefore beings fell from the sky (fallen ones) upon the earth. So when I look at your question I ask myself this, "What if aliens and angles and demons are one in the same." They are intelligent and not of earth. Oh man would that change people's religious views. lol However, it really shouldn't. If God is true, then details as to who, what, when, and where become irrelevant. But people would take that also as being reason to state God doesn't exist, instead of seeing the truth of the same thing explained two different ways.