It seems like you're missing my point, or just ignoring it. My understanding of what you're saying is there is no physical embodiment of God, therefore Santa is perceived as more real. Yes, I'm with you on the point that to children Santa is real because there is a physical embodiment. But the analogy between child / adult isn't even really apt, because children will believe anything you tell them. You could feed a four year old literally any story and they would believe it.
But my argument is that you're understating how much of a physical embodiment of the Adult God there is. No you can't actually see God, but you can spend your entire life surrounded by other people who literally believe in it's existence, and will do anything in their power to convince you that it's real. In some regions of the world people can spend their entire life coming across no contrary social inputs.
You can live in cities where there might be 20 - 30 churches, some very grandiose, and in Catholic versions very striking imagery. In these same cities there might be biblical quotes on a number of street corners.
The bible is the world's best selling book and at least half of the world's population believe that it's a holy text.
On Facebook you can follow religious pages that will promote religion day after day.
So for the adult it's not just a matter of intellect, and interacting with the world like a child, it's a matter of religion being a normalized part of our culture. Religion is even more pernicious than Santa because to most of the world it is real. I get that you consider yourself an activist of sorts with regards to religion, but the constant insinuation that the religious are just dumb feels off the mark to me.
I think it's more the case that those who can see beyond culture are very smart. Where those who fall for culture are just normal people.
I get what you are saying and I don't disagree with the gist of it. I can certainly see myself being the same way given the same inputs. It's why I don't try to change the way people think anymore than I try to change how tall they are. Would I be happier if I woke up tomorrow and observed that everyone had acquired sufficient knowledge to conclude that there aren't invisible people living in the sky? Certainly.
One of my favorite sayings is "You can't use knowledge you don't have." That is the explanation for why kids pretty much believe anything they're told, because they don't know any better. They simply lack enough knowledge to come to the informed conclusion that santa is a dog and pony show. Not until they muster sufficient knowledge does santa belief change.
How are adults that worship invisible creatures living in the sky any different? What exactly would allow them to "see beyond culture" if not knowledge? In short, they lack the intellect - lack the intellect for whatever reason. Those reasons are many and varied but in the end it's an intellectual shortcoming. And aren't very smart people normal people too?
That kid who believes in santa is living in his santa culture. Then it changes. The kid was just as normal before he stopped believing as he was after, but he's smarter because he has knowledge that allowed him to make an important change.
That same kid has no doubt heard about gods and religion too. Think for a second how many kids' lives would be different if told that some people believe gods are real and some don't. And that it's fine to be either way. Pretty important bit of acquired knowledge wouldn't you say? But how many kids have that experience? Pretty close to zero?
And that's why kids continue to believe in invisible people living in the sky. It's okay to stop believing in santa but it isn't okay to stop believing in invisible people living in the sky. That understanding, that knowledge, is denied in that culture.