Because it's factually wrong. Biology matters, and I'm not trying to bring in concepts like race (which is a total intellectual disaster, since the term refers to multiple concepts that mean radically different things). Many studies do not account for simple heredity, when adoption studies have already shown that traits like IQ and even personality are heritable to some degree (1). There are genetic correlations with intelligence, aggression, etc that are difficult to untangle but undeniably real.

Humans are incredibly unique animals, but you are still a biological machine and your brain is not a piece of magic pixie dust. Without getting into the free will debate, biology does not determine behavior but does influence it heavily.

Edit: I'm not denying the existence of culture or social norms. These matter, but are also undoubtedly linked to the biological machinery of the human animal. Human society is an evolutionary result (2).

Second edit: It's very easy to find evidence that the social evolution of humans is a biological process. Here is one of my favorite papers on the self-domestication of the human species (3).

1: https://www.amazon.com/Blank-Slate-Modern-Denial-Nature/dp/0...

2: https://www.amazon.com/Before-Dawn-Recovering-History-Ancest...

3: http://resources.seattlecentral.edu/faculty/jwhorley/Gracili...