One reason why this has attracted criticism is because data on hunter-gatherer societies are far and few between (and certainly not random), which makes it difficult to arrive at a definitive conclusion. One can only evaluate the available evidence and draw as reasonable conclusion as possible. Another reason (and I suspect a more important reason) is that it challenges romantic images of the past held by many on the ideological left. Although secularists often mock conservative Christians for believing in a literal "Garden of Eden," secularists cling to their own versions of the Garden of Eden, due in large part to the philosophical musings of Jean-Jacques Rousseau and Karl Marx. Indeed as the sociologist Daniel Chirot and psychologist Clark McCauley have noted, the parallels between Marxism and Christianity are striking:
Marxist eschatology actually mimicked Christian doctrine. In the beginning, there was a perfect world with no private property, no classes, no exploitation, and no alienation--the Garden of Eden. Then came sin, the discovery of private property, and the creation of exploiters. Humanity was cast from the Garden to suffer inequality and want. Humans then experimented with a series of modes of production, from the slave, to the feudal, to the capitalist mode, always seeking the solution and not finding it. Finally there came a true prophet with a message of salvation, Karl Marx, who preached the truth of Science. He promised redemption but was not heeded, except by his closest disciples who carried the truth forward. Eventually, however, the proletariat, the carriers of the true faith, will be converted by the religious elect, the leaders of the party, and join to create a more perfect world. A final, terrible revolution will wipe out capitalism, alienation, exploitation, and inequality. After that, history will end because there will be perfection on earth, and the true believers will have been saved ("Why Not Kill Them All," pp. 142-43, emphasis added).I don't know if it's something we're born with or something we acquire (or maybe a little of both), but it seems that many of us long for a past where all was pure and right and just. Of course, how we define what is pure and right and just differs from person to person, but that doesn't alter the fact that many of us pine for a golden age. And whether we do so in religious or secular terms is beside the point. The fact is that we do, and it apparently makes us resistant to data that may indicate otherwise.