Most people believe this because it is said twice in the Jurassic Park movie (the idea being taken from the book), but it is not true. It is somewhat true for amphibians with very simple visual systems and limited hunting strategies, like certain frogs, which would at least be an in-universe explanation for Jurassic Park's haphazardly cloned dinos. But in the movie Dr. Grant claims it for the first time before even learning of the existence of the park, so they don't get any points for that. In reality, T-Rex for example is believed to have had incredibly good vision - much better than humans:
https://bioone.org/journals/journal-of-vertebrate-paleontolo...