Ultimately they are not. It's just that the evidence points towards an extremely low chance of fairies being real and extremely high chance of others being conscious in a comparable manner to how you are conscious.
Every North American faerie weighs more than 10kg.
Every North American faerie weighs 10kg or less.
As another example, I suggest that “Every idea weighs more than 10kg” and “Every idea weighs 10kg or less” are both not meaningful, because it is not possible to measure the weight of a given idea.
And so consider a claim like, “Consciousness has property X”, how can I decide whether or not such a claim is true?
edit: That last claim should have been, “Some other person’s consciousness has property X”.