As someone who has dealt with ADHD both with and without medication, at least from my experience this line of reasoning leads to much pain.
I cannot speak for those with depression, but ADHD has easily tripled the amount of stress I have in my life, if not more. It causes impulsive behaviour that can wreck relationships(both personal and professional), and makes it extremely hard to concentrate on many things that aren't totally gamified from the outset.
There's a trope about how ADHD ends up being a gift because of creative thoughts coming to you, and that the medication will dull your senses... but I don't buy it too much. There have been studies that have measured no noticable difference in "creative capabilities"(hard to measure, granted) when being medicated and non-medicated.
Before being put on medication, I had never finished a single piece of homework in my life. Afterwards I was able to finish the homework assignments before class was even over. I would study autonomously, and was able to reach my full potential because I was able to manage my thoughts for the first time in my life.
There is no moral high-ground to not medicating oneself, just like there is no moral high-ground to using crutches when you have a broken leg. Asking me not to medicate is the same as asking me to go through meaningless suffering in almost every social interaction. Countless studies have shown that medication is much much more effective than any form of behavioural therapy (and that BT has an extremely high failure/"relapse" rate).
I don't think it's right to mock people looking for non-medical treatment, because various reasons mean you cannot have access to alternatives.
But medication has been proven to be very effective, and BT has proven to be a very difficult path... if you are a parent of a child suffering from ADHD it is your duty to at least explore the medication option in my opinion.
There is likely to be no higher reason for ADHD and depression to exist. We have looked at "normal people's" brains, and see that for depression/ADHD some things work differently. And that these people tend to have lower-quality lives because of a lack of something... It's hard to argue that there's a reason for it all except in some religious context. It's not like there's a higher reason for brain tumors either.
As an aside, despite the fact that I currently love where I live (Japan), I always consider moving back to the US if only to get access to medication that is illegal here. My life is a lot shittier (still great overall though) because of some drug laws. I have spent a lot of time working on different non-medical coping techniques that have helped out a lot, but nothing matched the chemicals.