Revealing mistake: In the scene where Evan wakes up and notices that he has no arms, right when you see the stubs you can see a faint shadow moving next to his head and moving on the headboard. This shadow is the shadow of his hand before it was digitally removed. (00:29:15)
Revealing mistake: Although the film is supposedly set in New York, there is a Canadian speed limit sign in one of the first shots. Across the street which Evan lives on.
Revealing mistake: In the first scene of the film, Evan is writing frantically in a journal and his last words that the camera zooms in on are "Save her". Look closely and you can see that those same words are already indented on the page as indicated by a visible letter "s" and "h" and a period that had already been written on the pad of paper. Obviously they had done some other takes. (00:01:35)
Answer: The point of the movie was that, no matter what he did and how he tried to change things, they always ended up bad. If he went back and did that, something unforseen would have happened to make things terrible. Evan realized that everything bad that happened to them was because of him. He then decided the only safe way to make things right is if he just took himself out of their lives all together. That's the logic the filmmakers went by, if you don't want to accept that, then you will just have to consider it a plot hole.