because it requires active eye contact. dead people can't do that.
also, the 3d mask with 2d eyes required many attempts to align it properly for it to work at all. they didn't show you all of the failed attempts leading up to the one that worked.
they also knew the passcode, so whenever it failed (which it did), they could unlock it and keep trying.
a dead person can't tell you the passcode, so once it locks, game over.
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