Revealing mistake: When Tim Allen is looking at the reindeer, one reindeer's ear moves. At this point you can see the metal robot in the base of the ear.
Revealing mistake: When Santa slips on Scott Calvin's snow-covered roof, you can see the "snow" wrinkle as he slides.
Revealing mistake: When the elves are tying up the police officer at the station, they do so in a really quick manner. The film here is obviously sped up. You can tell this by the fact that the children playing the elves are breathing really fast.
Revealing mistake: When the original Santa falls off the roof, the path he slides is already visible before he falls, especially in the point of view shot.
Answer: If he told people he was not deliberately trying to look like Santa Claus, that would cause them to be even more suspicious of Scott Calvin having a mental disorder, not a physical one. Remember, Scott went to see his family doctor, Pete, and Pete tried to explain to Scott his physical changes as a matter of changing his diet from milk and cookies, and Scott suffering from a hormone imbalance. Scott tried, but not even Doctor Pete, a professional in the medical field, could help him.
Scott215
This question is about his mental state. Not his physical transformation. In other words, he could say "There's nothing mentally wrong with me that would make me think for one second I am Santa." His physical transformation could be explained by people thinking he's overeating, growing facial hair and his hair turning white.
Except, Doctor Pete was incompetent. Nobody has the drastic physiological changes that Scott had, and Dr. Pete seems committed to blaming them on 'routine' aging and diet factors. In real life, a competent doctor would be submitting Scott to a continuous battery of tests, cancer screenings, CT scans, etc.