Why do some people believe the human eye is an example of why evolution cannot be true

Because few things about the human eye have changed throughout the course of human evolution. The entire human race learned to become truly bipedal, yet after millions of years, we still cannot see in the dark (like actual night vision). We still only have three cones/ color receptors in our eyes, the same range of unique eyes colors (controlled by melanin), shape and disorders. The human eye never even grows during your life; it is the same size when you are born to the day you die, with the exception of some deterioration. The nonexistence of change is why it is not considered evolution; to humans, there was no need for change as we had developed other ways to survive.