I swear someone asked this question a long time ago...anyway, I think there are a couple reasons. The first is that anime was originally inspired by 30's-40's cartoons and movies from the U.S. by Disney and others that they were introduced to during the occupation. Interestingly, anime may have never became a thing if Japan hadn't been nuked.
The second is that most all Negroid (Africans) and Mongoloid (Asians, Native North and South Americans, Pacific Islanders etc.) cultures the world over have a very homogeneous look. Most all have very dark to black hair and dark brown eyes. However, the Caucasoids (white people) of Eurasia have many different eye and hair colors from black hair or dark brown eyes all the way to light blonde or red...