Why do we not treat this as an ill?

Much of it comes from the same place. Depression, ADHD, and many others affect our ability to concentrate and plan. When you are in those situations, long term doesn't matter - you are fighting to live in the short term. Part of using medication is helping to get those effects under control so that you can be forwarded looking and disciplined.

Simply saying "exercise more" to someone struggling with mental health is like telling a person who is bleeding to death that they should stop bleeding. Yeah, chances are they really want to, but they don't have control over it in the moment.