Good point. I suppose it is because my mind is equipped rather poorly to handle the concept of infinity, and it is far easier to make decisions based on past knowledge than on evaluating infinite probabilities. It's not that belief A has a greater or lesser probability (in the strict sense) than belief B, just that all my experience confirms belief A and not B. If, as I think you said earlier, there an infinite number of possibilities, then there is no difference between a 1/infinity prob and a (really big #)/infinity prob. It just does not make sense to take the probabilities over the infinite set.
Then why do you make judgements based on this reasoning every moment of your life?
Why for example are you not cowering for fear that the five foot, six armed dragon, standing to your left, is about to eat you?