Because it is a fallacy to attribute to nature what is clearly at least significantly product of social relation. This is one of the very interesting things everybody (no matter their political affiliation) can learn from Marx and the whole group around Adorno and Horkheimer, but you will find this echoed by more conservative sociologists and historians, too. Zizek also frames this quite nicely in The Perverts Guide to Cinema, where he explains, that ideology is never a pair of glasses [EDIT]you wear[/EDIT] but the very eyes through which you see. Right now I sadly cannot look through my books to find you less leftist references for the topic. If you are interested I can link to some less controversial works of sociology once I have access to them again. (Edited for some typos and the marked addition.)