Because there are only two animals that I know of that are above humans on the evolutionary ladder: cats and dolphins. Cats never got over being worshiped as gods in Egypt and dolphins, well all they do is eat, play and have sex all day. You tell me who is smarter. Cows exist to be slaughtered (except in India where they are not slaughtered for fear someone might kill someone's grandmother's reincarnation). They must have bad karma.
Actually, humans can only be humans, cows cows and cats cats. Dolphins can become gods though. After thousands of incarnations, humans, through discipline and wisdom, become dolphins. That is my goal.
assuming you are being serious about this......... how do you know all this?