Because "non-toxic" finger paint might not really be non-toxic
If you have a little artist who's still too young to know that paint isn't food, you might want to consider making your own safe and edible paint. It might not surprise you that paints and other art supplies labeled as "non-toxic" might not really be non-toxic. Unfortunately, there's no real guarantee what's in your paint because most ingredients in commercially available paints don't have to be disclosed.
However, scientists do know that pigments used in paints can contain toxic metals like cadmium, lead, and nickel. And preservatives need to be added to a lot of water-based paints so the product can sit on the shelf and not rot or mold before being used.