You are misunderstanding the American building system. We don't use fiberglass insulation by itself. You're picking a small part of a greater system and complaining it doesn't work in isolation. There are multiple layers to our construction and it's effective.
As for the wall material stopping drafts in a window frame, that makes no sense at all.
Exactly, the critique is that you need multiple layers, and if one fails, the R values you thought you had go out of the window. Whereas a concrete, wattle and daub etc wall will tolerate less-than-perfect fitting and maintenance. Of course if you assume construction is flawless, there will be no issues, and that’s the usual case.
As for the wall material stopping drafts in a window frame, that makes no sense at all.