The chance is locked in because, by the rules of the game, the host can't open that one and show you anything -- regardless of what it contains. You know that t

The chance is locked in because, by the rules of the game, the host can't open that one and show you anything -- regardless of what it contains. You know that there's at least one door that the host can open (of the two remaining doors), so them opening a door changes nothing about the door you've chosen.