Well, there are, of course, a number of reasons, but perhaps the primary reason is because we're still organized as a species in the same way that we were organized 200 or 300 years ago.
Их, конечно, немало, но главная, видимо, в том, что как биологический вид мы до сих пор организованы точно так же, как и 200-300 лет назад.