Here's why:

* Vitamins are already in their simplest form: Unlike carbohydrates, proteins, and fats, vitamins are not large molecules that need to be broken down into smaller units for absorption. They are already in a form that can be directly absorbed by the body.

* They are not broken down: Enzymes are biological catalysts that speed up chemical reactions. Digestion involves breaking down food molecules into smaller components. Since vitamins are already in their simplest form, they don't need to be broken down by enzymes.

How Vitamins are Absorbed:

Vitamins are absorbed directly into the bloodstream, primarily in the small intestine. The process involves:

* Fat-soluble vitamins (A, D, E, K): These are absorbed along with fats in the diet and require bile to be properly absorbed.

* Water-soluble vitamins (B vitamins and vitamin C): These are absorbed directly into the bloodstream.

In summary: Vitamins are not digested by enzymes because they are already in their simplest form and are absorbed directly into the bloodstream.