An agnostic about the existence of God says that he cannot know whether or not there is a God, so it is sensible to withhold judgment on the question. Assuming one is agnostic regarding God, yet does not believe that entities like space aliens, ghosts, leprechauns, fairies, etc., exist, then the question arises of why disbelieve in the latter and not the former? If the agnostic answer is that space aliens et al are clearly man-made fictions, then well and good. (Putting aside for argument's sake that examining the origins of religion also show it as a similar man-made fiction) I wonder if there is another factor at work in not disbelieving in God.
Note that space aliens, ghosts, leprechauns, fairies, and the like are of little importance to men. They interfere in our affairs for good or bad, but only to a limited extent. Not so God. God made the world, gives to life morals and meaning, and provides an afterlife for us. Unlike the leprechaun and his counterparts, God is of utmost importance to men. God, if real, is fundamental to the universe and our lives.
Is the agnostic's hesitance to disbelieve in God due to the fundamental importance attached to God?
God's importance is of a philosophical nature. To disbelieve in leprechauns is to - at no great cost - exclude them as a reason for one's good or bad luck or just as a curious creature from one's understanding of reality. Is the philosophical importance attached to God an impediment to disbelief for an agnostic who does not believe there are leprechauns? The existence of leprechauns is intellectually easy to dismiss. Perhaps for some the existence of God is not so intellectually easy to dismiss solely because fundamental intellectual matters are by definition attached to His existence and with that comes the unpleasant realization that these important matters would be dismissed with Him - which is nihilism; but not being nihilists, they settle on agnosticism. In other words, the importance of an alleged supernatural being is related to withholding disbelief in it. "Afterall, God is important, so maybe He exists." Is it rational to hold that one unimportant supernatural being does not exist but an important one may or may not exist?
I raise this question because I often sense this is implicitly in what agnostics argue. Then again, maybe it is just me reading too much into their positions. I don't know.
If one is agnostically stopping short of disbelief in God as a result of this supernatural being's importance, here is a thought experiment that might clarify. Imagine a leprechaun, not as a mere mischevious and secretive magical creature, but instead imagine the leprechaun is the creator of the universe, giver of life and morals. Then the leprechaun becomes God.
Is withholding judgment on the existence of that God rational? Hardly. Same for any God, goblin, or ghost. The point is, the importance or non-importance to mankind of any supernatural beings is not important in reasoning to the conclusion that there are no such beings.