Why the Disappearance Is the Whole Joke
The door is not the joke. The frame going quiet for one person is.
A lineup is just setup
Five people standing in a row is not funny. Five people standing, then four, is. The whole clip earns its reach in the one second the frame loses a person - everyone's attention snaps to the empty spot and the two friends on either side look completely lost. That sudden gap is what people rewatch. The rig, the escalator, the floor: those are just the vehicle. The moment is someone no longer being there.
Why it works on so many surfaces
Because the joke does not depend on where it happens. Elevator, escalator, staircase, classroom floor, office hallway - none of that matters. What matters is a row of people and one of them dropping out of it. That is why the trend kept hopping from place to place: the disappearance is portable, the location is not. The Trapdoor Challenge keeps the portable part and drops the set.
The reaction sells it
A clean vanish with the others frozen mid-laugh is the version that travels. The empty space plus the delayed "wait, where did they go" is the punchline. In the game, that same beat is what your prediction is graded against - did you know who would be gone before the row lost one of its people.