so we now have the original door, and the one remaining door.
that's a 50/50 chance. all the removal of the known-incorrect doors has done is increase the chances of the initial choice being right.
The random event is actually the placement of the prize. Imagine the organisers throw a die to decide where to put the prize. Whether or not there is a prize behind the door you chose depends on the die thrown and provided there isn't some shuffling of the prize behind the scenes during the competition, that never changes. How could it? The car, the compartment it is in and the door are all totally unaffected by by opening or closing other doors.