It's based on the same concept as wild animal psychology. Have you ever met a wild animal that, from birth, has been "evil" or bad-natured? No. It simply doesn't happen. Some would argue that such animals as hyenas or sharks are "evil" based on their cruel, killing instinct, but that is simply how they are engineered to survive.
The reason is that "Evil" exists at all is because of personal experiences, chemical imbalances in the brain or stimuli that is not native to the body.
It is for that reason that evil Pokemon cannot and do not exist in the wild because they have no cause to be evil. There is no outside force that influences them to be so.