The thing that you're missing is that most health professional will still include things like "exercise and yoga" in their treatment plans because that's what is required. Usually it's the patients that expect pills and therapy to show magical, immediate results. Nobody (well, I'm sure there are doctors that would, but at the risk of invoking a No True Scotsman, some doctors are pretty bad at their job) is going to tell a depressed person or a person with severe ADHD to just pop pills and not exercise, meditate or otherwise not do any self-care. The pills are there to help the ill person get to a place where they can start working towards those things, pills are step one of a long process, they're not the only step. The thing psych meds are not taken with much skepticism is that they can show results incredibly quickly, even if getting the right results cant take months of fine-tuning and trial and error due to the fact that the mechanisms aren't well understood.

A very good analogy for mental illness and psych meds is diabetes and insulin. Most people who are prediabetic or have diabetes will have type 2, adult-onset diabetes, which can be stopped or reversed with lifestyle changes, but those are hard. If an obese person is feeling terrible and goes to a doctor and gets diagnosed with diabetes, the doctor will give them insulin because they need it to live and function, but it's not actually going to solve the problem. Insulin is just step one for such a person, step 2 involves exercise and diet, maybe surgery in severe cases, but many people can't commit to such lifestyle changes and just keep taking insulin as a stop-gap measure to keep living in relative comfort, while other people will manage to use their new feeling to take up the needed lifestyle changes and get cured (or go into remission, as they say), possibly to a point where they no longer need insulin or other medication.

In the same vein, just as there is type 2 diabetes, there is type 1 diabetes, whether it manifested because of environmental factors or was purely inherited, these people need insulin to live. The state of medical science today means they will need insulin and other medication for the rest of their lives, but they still need the diet changes and exercise, nobody is expecting them to just sit on medication and nothing else.

With mental illness it's the same way, some people can get better with just exercise and lifestyle changes, but some people need professional help (meds and therapy) to be able to implement those lifestyle changes and maybe get to a point where the no longer need the help. Other people, however, will need meds for the rest of their lives to have a chance at normal life, at least with the current state of medicine. One can hope for a breakthrough in the future but people need to live their lives right now, and if that involves taking a pill or two every day, so be it.

In my particular case, I think that I was pretty lucky that me and my psychiatrist only took 4 or so months to find the proper combination of meds for me, but at no point did he tell me that was all I needed. He always insisted that diet, exercise and mindfulness were part of the treatment and the meds were there to help me get started. Now it's been roughly a year since I started and I lost a ton of weight, exercise regularly (including yoga) and eat healthy. I'm down to only taking my ADHD meds because the others did their job and I don't need them anymore, but even if I have to take this pill daily for the rest of my life I'm okay with it because my quality of life skyrocketed. This was not possible for me a couple of years ago when I thought that I could power through my issues with yoga and exercise alone and didn't trust psychiatrists.

I think it's incredibly irresponsible to tell people that things like yoga, meditation, exercise and diet changes are a replacement for professional mental health help. This may be true for some people, but I suspect the percentage is close to the amount of people who are able to stop being alcoholics without help. The two things are orthogonal, professional medical help is one thing and a healthy lifestyle is another. Having a healthy lifestyle may mean that you don't need professional help, but professional help may be required for a person to be able to get a healthy lifestyle. Finally, professional help usually involves all those nice things like plenty of exercise and a good diet, sometimes yoga or mindfulness/meditation (even if they call it something else the principles are usually the same, CBT for example is basically mindfulness by another name). You go to the a professional so he can help you get going with all of that, because sometimes you just can't by yourself.