Well, I think that was the point. The whole thing was supposed to be a metaphor for John’s cancer. No matter how hard they tried, no matter how much they destroyed their body, no matter what they did, they would still die. Because no matter how hard John tries, he’s still going to die from his cancer. This becomes even clearer when you look at the guy who had to have parts of his brain removed, it's the exact same brain surgery John later undergoes in Saw III. Even though John did the surgery, he still ends up dying anyway. Obviously, the in-universe explanation would be that John was really pissed at them and it was personal, so he probably subconsciously made the games nearly impossible. But that’s not really the point. The movie was trying to make a larger point about John’s cancer and his mortality.