There are several reasons. Medical-grade skin care products have been designed to penetrate the epidermis and affect deeper layers of the skin. Over-the-counter products do not contain the higher concentration of ingredients therefore, they are designed to sit on the surface of the skin.
Additionally, a major difference between products offered by licensed professionals versus drugstores is that medical-grade skincare is under FDA regulation to contain 99.9% pure ingredients and to ensure safety and effectiveness. They are supported by scientific research and are specifically formulated for best results.
Finally, Medical grade products are much more effective because while the price may be considerably higher on medical grade products, they are actually more cost effective because with more effective and highly concentrated ingredients, you will be able to use less of the product and get better results.