1) There is a well established framework of understanding strongly supported by physical evidence: cookies have been nibbled in the morning, presents under tree as predicted, visual sightings, etc.
2) The Santa story answers some important questions and brings up many others: How big did you say his bag was?
3) The story is falsifiable: as soon as we have confirmed evidence of Christmas eve cookie eating of non-Santa origin, we may need to adjust our theory. We don't know of this evidence currently, but we remain open to this possibility.
4) Santa's validity is not established by popular opinion, but objective reality: other people may have different theories and they are always welcome to present the evidence for these alternative ones. Teasing or ridicule does not convince us."