That's because people are shitheads and I hate them.
Affirmative action is simply the implementation of the view that society should be comprised, in as many areas as possible, of demographics which reflect the demographics of society as a whole - ie that prejudices should not be allowed to dictate the construction of the institutions which rule our daily lives. It does not 'other' anyone - it welcomes them into areas previously closed off. And the principle would, in theory, defend a white minority same as a black or Asian minority. It is a way forward, a better world, a more united world, not a less united one.