Well, starving oneself is not a feasible method. It's brought up on this forum here and there, and everyone, except the person bringing it up, knows that it obviously won't even come close to working. It just won't.
Jumping is "foolproof" in the same way that full suspension hanging is "foolproof" -- if you do it right, it is guaranteed to kill you. And it's easy for any able-bodied, non-institutionalized person to do it right. That's not where the difficulty lies. The difficulty lies precisely in the fact that these two methods offer certain death with no possibility of reversing your decision once you've taken the final step.
There's a lot of talk on the forum about partial suspension hanging. And the extremely high failure rate associated with it. It's a go-to for many here, precisely because it's not a sure thing. It *might* work, but you don't have the absolute certainty of death that you do when you kick a chair out from under yourself. That's just how it is -- most people have fear, uncertainty and doubt about ending their life. Sliding towards it, having the option to abort the attempt midway, is more palatable.
What people lack isn't foolproof methods, it's fear-proof methods. None exist.