Well, partially, because it’s hell on the first responders to have to clean up a fatal accident scene, which is made worse if the death(s) could have been prevented.
I don’t disagree, but you are making the case for regulating grown adults. For the parent post “who decides how long the waiting period should be?” The answer is society. Right now society says none, but it shouldn’t be looked upon in horror if that changes. If I had to choose, I’d choose seat belts optional over on-demand euthanasia.