A random holiday thought

I love Christmas movies, but one thing always bugs me about them. You know how Santa Claus usually turns out to be real after all? Why are the parents always surprised by this? If he was real, the parents would have to know, because every year (and not just the year that Santa Claus is "proven") there would be presents under the tree that they didn't buy. Okay, maybe some people are absent-minded enough that they might think they just forgot about buying that particular present, but certainly not all of them. The "Santa Claus is real" plotline, as enjoyable as it may be, just doesn't work when you think about it logically.