Because the basis of leftist ideology is that people are good in their natural state and culture ruins them. They project this on to archaeological finds. Read the intro to Lawrence Keeley's War Before Civilization on Amazon (buy the book, too, but you can read the intro for free). His story about "fortifications" and "enclosures" is quite interesting.
Rousseau, the "Noble Savage." Another pretentious, screwed-up Frenchman.