Many of the vitamins and minerals are cheaper to produce at high quality with industrial production, such as ascorbic acid (around $12/kg - https://www.foodbusinessnews.net/articles/10836-vitamin-c-pr...). The dosage can be more precise in this way.

Also with diabetes and certain other illness, some nutrients can't be taken at a required level from natural sources due the amount of other nutrients, e.g. carbohydrates.