Continuity mistake: While Charlie tells Scott he knows CPR, Scott's right hand drops to his side, but when Charlie tells Scott the "SC" on his pyjamas is the same letters as his name, Scott's right hand is now resting on his right breast.
Continuity mistake: After Santa falls in Scott's garden, before he vanishes he has some snow on the suit, but a few moments later the snow is gone.
Continuity mistake: In the first house visited by Scott as Santa, the amount of snow on the top of the chimney changes from shot to 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.