That stems from the emotional side of things, which explains why some of the people here won't let the erroneous thinking go. They don't want to be wrong in their initial pick, so it's emotionally difficult to let it go.
Jaden
The essential logical error being made by the unbelievers is assuming that if there are 2 choices it is 50-50. That would be true IF the 2 choices had the same likelihood, but they don't.
I've been watching Daniel Negreanu's WSOP vlogs and they contain a perfect example of this. As he drives to the casino he always encounters a particular traffic signal and it is almost always red. He enquires further and learns that the light has a red cycle of 120 seconds and a green cycle of 12 seconds. Which means it is ten times as likely that it will be red than green. Which accords with his experience. But according to some of the illogic we've seen, some would argue that there are 2 possibilities, red and green, and therefore it is 50-50.
Back to the doors -- once you've picked your door there is a 1/3 chance you are a winner and a 2/3 chance the winner is in one of the other 2 doors. We know that either way there is a loser door among the 2 unpicked doors, and Monty knows which (or both) of the 2 doors is a loser. So when he shows a loser door he is not adding any information. There is still a 2/3rds chance that the winner is among the unpicked 2 doors. And with the exposure of that loser door that 2/3rds chance is now distilled down to the other unpicked door. NOTHING has occurred which would change the 1/3 chance that your original pick was the winner. So while there are now 2 choices, one of the choices (your original choice) has a 1/3 chance of being the winner and the other choice has a 2/3 chance of being the winner. So by switching you double your chance of winning. Just because there are 2 choices doesn't make it a 50-50 bet because the odds of each choice being a winner are not the same.