I think it is an interesting question, if not a particularly deep one. The answer, it seems to me, is that the force is not directed against the building as a whole, but against individual lower levels, which collapsed in sequence and "recruited" as part of the downward-pushing mass.
Yeah, it is an interesting question. I had to think about it for a bit. I think the main issue is simply that the car/truck example and building example are very
different systems and so can't be directly compared. For example, the building as a whole experiences a constant gravitational force in the direction it is collapsing, the car/truck system does not. The building has a relatively uniform density with plenty of "crumple points" (floors), where as the semi has a nice solid engine in the front which is colliding with a basically hollow small car (in the photos).
A very important point is that material's strength does not
scale with size. I recently watched a video (if I find it I'll post it) where a person modeled the building with 7 or 8 stacked paint cans, dropping the top one from a few inches onto the bottom ones, "recreating" the same scenario outlined in the image. Unsurprisingly, the paint cans did not "collapse". He scaled the size
of the building properly, but not
the material's strength. I see this all the time, when people try to recreate the collapse scenario using small scale models. In this case, I have no idea how the material strength and construction of automobiles compares to that of buildings.