Is it a plot hole in many Santa Claus movies that no one believes in Santa, yet he turns out to exist in full regalia?

“Plot hole” is an understatement.
We are talking about a being that breaks into homes and leaves commercial products there.
It is impossible that none of the millions of adults notice that toys that they never bought are suddenly popping into existence on christmas day.
Plus, where does this Santa being get all the toys? Either he buys them all, which would mean that all those companies have records of the annual sale to a singular buyer, or he builds them himself with his elf-slave labour, in which case the toy manufacturers would all be on his arse, trying to nail him for his product piracy of a scale hitherto undreamt of.
After collective lobbying to enact anti-santa laws, on the 24th the streets would be crawling with cops looking to catch santa in the act.

People will believe in anything, Santa, God & Trump will never be president.

Santa doesn’t exist, and everybody knows it,

Bullshit, I say!

Santa doesn’t exist like Uncle Jojo wasn’t gay, even though he never lived with the French wife who needed a visa, and his “roommate” Michael, who didn’t have his own bed, always came along for holiday dinners.

Everyone knows Santa exists, they just won’t admit it.

The answer is right inside your question: they are “Santa Claus movies”.
The movie industry does whatever it can to make money from the production of any endeavor. Think how boring the movie would be if there were no, however much expected, plot twist. Who would spend money to see it?

Here’s the plot for a Santa movie never to see the light of day.
A 7 year old girl sends a letter to Santa asking only for her mother to recover from Stage 4 breast cancer. She decides to write a letter and pray every day.right after her father opens the little door on the Advent calendar her mother gave her. The wise, old, neighborhood postmaster notices the daily mailed letter and after a short investigation decides to help. He asks his contacts to write letters as well. They ask their friiends and they ask their friends to do the same.
The night before Christmas, the mother throws a massive blood clot and suffers a major stroke resulting in her death.
The End…. Roll credits as the ciamera slowly pans over a mountain of letters in the central post office’s S.C Box.

Think any major producer will pick this up? Me neither. .

@zenvelo I was referring to the plot of the movie. In the movie, everyone, parents, bosses, friends, all agree that Santa does not exist and everyone who believes in Santa is regarded as a nutcase. But then, Santa turns out to exist after all; North Pole, delivering presents all over the world.

If Santa really exists sending presents all over the world (in the universe of the movie), then, how can nobody in the movie believe in him or know about him, besides small children and loveable dullards and crazy people?