convergent evolution

Evolution.

The eye is a good example because there are living animals with eyes that represent each step in it's evolution. Human eyes also show how evolution can produce "bad" results; the retina is essentially "inside-out" - the nerves are on top and there is a blind-spot where the optic nerve leaves the eye.

Yes. For instance, vertebrates and octopuses evolved a very similar type of eye, independently from one another.

Mainly because they do not understand the process of evolution, or they refuse to believe it on fundamentalist grounds.