Some people like the dark ages. The advantage goes to aggression and physical strength. Some would rather compete in such a world. It could be why they seemingly battle against civilization.
I don't care much of a badass anarcho-primitivist some modern person thinks they are. They may think they would rise to the top like Genghis Khan if only they were free of the straightjacket of civilization and its morality, but almost all of them would really starve to death after a winter or die in a ditch of sepsis.
[1]: https://en.wikipedia.org/wiki/Collapse:_How_Societies_Choose...
Your link is interesting, I didn't know about that book!
I don't think that's true: https://slatestarcodex.com/2017/10/15/were-there-dark-ages/
Of course, I don't claim to be an expert myself and can't readily judge which of the two articles is more accurate.