I'm going to have one more try for FA. Let's try a modern skyscraper, because it's easier to explain than a steel-framed one.
A modern tall building is held up by its core. If you've watched them being built, you'll see that the life shaft is put up first, then the floors are attached to that, then the walls are attached.
Each floor is the same thickness, because all it has to do is support its own weight, and the weight of its own wall.
Unlike a cathedral, where the walls are thick at the bottom to support the weight of the thinner walls above, the walls are a constant thickness, because the wall of each floor is supported by the floor. There isn't a continuous wall running from the top to the bottom of the building; rather, each floor has its own wall, which is the same weight be it the bottom floor or the top floor. So you can remove the wall around the 20th floor, and the 21st floor walls don't move, and no load is removed from the 19th floor walls.
This is pretty critical for buildings with a lot of glass, because although you can make glass structural (the windscreen on a modern car, for example, is structural, as you'll know if you've ever driven a car with a broken windscreen) it's expensive and difficult.
OK so far? You've got a lift shaft, with some floors coming out of it, with a non-structural wall to keep the rain out resting on each floor. The floors are structural insofar as they support their own weight, plus the weight of one storey of wall. The walls aren't structural at all.
Now, imagine a floor fails. It breaks away from the core, and drops onto the floor below. Now that floor is supporting two floors' worth of weight, which it might be able to do. But it failed for a reason, and that reason may be (as happened on WTC) a fire. So a couple of floors drop, until finally you have enough weight to fail a floor because of the debris sat on it. If the debris from above was enough to cause the 20th floor to fail, that debris plus the debris of the 20th floor will certainly cause the 19th to fail, and so on down.
The walls won't provide any resistance, because they are simply there to keep the rain out: the walls are designed to support their own weight, like the walls of a single-storey pre-fab hut. Actually, not even that, because those have to support the roof. And so it goes down, leaving just the lift shaft behind. It drops into its footprint, because there's no lateral force: each floor is failing and dropping onto the one below. It doesn't matter if it fails slightly asymmetrically, because the failure of each floor below will tend to damp out and unevenness.
Now a steel-framed building is slightly more complex, but the same basic analysis holds. The structure is a vertical bearer with floors cantilevered off it, with the walls just decoration at the edges. The structure is more diffuse, so rather than a single concrete lift shaft there are girders forming the core, set in concrete. But again, each floor only has to support itself and its wall, and each wall only has to support itself.
People keep thinking of sky-scrapers as like very tall houses, where each wall is supporting both the walls and the floors above it. It's not like that. There's a single load-bearing structure which takes the load, and then each floor and wall unit is just a floor and some sheeting on the outside. Take the walls away of a house and the floors collapse to the ground. Take the walls away from a skyscraper and nothing happens. It is, literally, just a stack of floors, and those floors are no stronger in a building of 100 floors than a building of 5: each only needs to support its own weight.