[headsupftw]

To me the key to understand the problem is paying attention to this detail: Monty knows where the car was so he wasn't just randomly opening A door. He always opened a door that didn't have a car behind it. It's this non-randomness that's giving you more information about the remaining door.

[moosinho]

Not exactly, if Monty randomly picked a door and happened to open a goat then it would still be beneficial for the player to swap.