Previous choices are moot. They are gone, they do not exist. They do not impact subsequent choices

But they do here, gnu. If, for example, your initial choice is Door 1, the host can open either Door 2 or 3 to reveal a goat. Let's assume he opens Door 2. You can then choose between Doors 1 and 3.

If your initial choice is Door 2 and the host opens Door 1 you can then choose between Doors 2 and 3.

In other words, your initial choice has influenced what your subsequent one can then be. It is no longer a simple 50/50 choice.