Why do some people believe the human eye is an example of why evolution cant be true?

People might think that the eye is too complex for evolution to explain. Natural selection couldn't have created it because no single part of the eye does anything on its own even though it has many seperate parts.

(As for whether it disproves evolution, the creation of the eye is something that's hard but not impossible for evolution to explain. Say a spot sensitive to light on a primitive species' skin would work sort of like echolocation to avoid predators, which would work into natural selection. A depression in that patch similar to that of a planarian could have developed, and then in future species it would fill with water which focuses the light, and then a lens to focus even further, and cones to distinguish colors ... etc.)