Because it is complexity. In Pro/E and SW/SE everything is separate. So the drawing doesn't need to update it's own because that is placed inside a different file. That file is only updated when you open that file. And the drawing is also stored inside that file, not inside another file, so you don't get all the information stored in ONE file but stored separate.
About assemblies, well I trust Zolko in his opinion, but that should also only be an assy, with volumes and curves, csys, lines etc, but not with solids. This also counts for Sheet-Metal Cuts for instance. Each operation in an assembly should have it's own assembly. Otherwise it becomes a mess.
With parts and bodies the same thing. A part should be a part and that is it. When you have instances that should be an instance and that is again it.
That way you separate everything and it becomes much more logical, but FC allows everything. That is very much like the NX way of working, but also at NX they start to realize that having all this features might not be the best thing to do.
I am not talking about that. I am talking about reducing complexities by separating it. When you only allow ONE body inside a part it becomes much easier to handle the mind. The same counts for assemblies and drawings.It certainly would not reduce disk I/O (probably increase it actually).
Each Sketch associated with a model would need to be accessed for every recompute. And, every change in a sketch would mean accessing ever other file in the model.