Well, it is simplistic from a certain point of view. If we rewind to the mid-to-late 80s with the Lisa and original Mac, which do you think most users at that point would find more simple and natural: windows that act as analogues of physical papers on a desk, or windows that have no real life equivalent and resize to fill empty space under arbitrary conditions?

Granted, that was ~40 years ago and things are different now, but you can see where the roots of the idea trace back to, and the idea probably has some level of relevance even today. As noted in my other comment, on Windows users often opt out of window management entirely by maximizing everything because it's simpler.

> it's hard to pick out your foreground window from 20 other background windows

This is mostly a casualty of two things: first, the flat UI epidemic which stripped contrast from UIs and made active/inactive windows states less distinct from each other and second, the proliferation of web/Electron apps which for the most part do not implement inactive states for controls at all.

If you boot up a Mac running the last non-flat OS version (10.9 Mavericks) and do work on it with entirely native apps, you'll find that the active window is very visibly active and background windows are very visibly inactive.