I can't speak for everyone, but regularly exercising and eating healthier foods reduced my need to take my ritalin as often. When I first started taking it, I had to take a dose every 2 hours to stay on track. Once I started eating healthier and doing ~1 minute of exercise every hour, I can comfortably go 3-4 hours between doses. I still function much better when I take my medication, but as long as I eat healthy and exercise, I don't feel useless without it.

Yes, exercise absolutely helps, but a person with ADHD is much more likely to exercise if they have been prescribed medication. Without medication they will likely just stop exercising at some point, because they "don't feel like it today" and then forget that they were supposed to exercise every day. Then 3 months later they remember for a moment, commit to exercising again, but that only lasts for another 3 weeks.

You're probably right when it comes to those of us who medication works well for, but there are some people who medication doesn't help much. In those cases I think diet and exercise tend to be the best line of defence against symptoms.