Sure, if Mommy walked behind a wall, you had actually seen her. You had real evidence that she exists. But if you don't have the slightest shred of evidence that fastastical beings like Santa Claus, leprechauns, unicords or gods exist, then it's only logical to assume that they don't.

What's really infantile, if you ask me, is leaning on an invisible friend/father figure during times of trouble rather than taking responsibility for your own life and your own problems.