To understand this, you just need to determine what a disease is. If it is pain or a malignant tumor, a blood clot in a vessel or a destroyed joint, then dietary supplements will not help.
And if you admit that you have to deal with symptoms when you have not paid attention to the causes of pathology for many years, then dietary supplements even help very much (again, with systemic administration). For example, detoxification, improving digestion, protecting cells of the nervous system from damage, improving the sensitivity of cells to insulin in diabetes are quite possible.
The main thing is not to look for magic remedies, but to study the mechanisms of action or consult with an expert on the use of the same dietary supplements. I have been dealing with this issue for over 20 years and I can say that I use drugs in my family very, very rarely.
And some people recover without taking any medications or dietary supplements. Others drink whatever they can, but they continue to get sick.
Supplements contain natural bioactive substances, which, depending on the composition, help to restore the body's strength in various diseases. There are dietary supplements that act simply like vitamins, and there are those that really help get rid of the symptoms of diseases. The composition of such dietary supplements contains analgesic and anti-inflammatory components, glucosamine, etc.
Vitamins are dietary supplements. If people with hypovitaminosis drink vitamins, they will recover.
But if people with lung cancer drink only vitamins, they will die. Because dietary supplements are not a cure for lung cancer.
The statement of your question hints that you already know the answer and my opinion will not change anything, but I will try.
To begin with, the fact "dietary supplement is not a medicine" means only that the drug did not pass (not necessarily did not work, maybe did not pass) registration as a drug in this country and, as a result, has no "drug" restrictions on sales.
That is, this does not mean that the dietary supplement has no benefit.
The second interesting thought, similar to the first, is that surgery is not a cure either. And exercise therapy. And ....
The third thought is that dietary supplements are very diverse in nature, and some may work, while the second may not.
Fourth, the effect should not be disregarded placebo
It depends on what disease and what kind of ailment. To put it simply, medications treat a serious illness, while dietary supplements treat malaise. For example, you have insomnia. Its mild form can be cured by taking soothing herbs, if the form is severe, then sleeping pills and even tranquilizers. Vitamins are considered dietary supplements, but many of them can greatly help a person. For example, we will not consider brittle hair and nails, dry skin a disease, but we will take fish oil to remove it.