There are several reasons why designers may choose to use a custom license with royalties. One reason is that it allows them to earn ongoing revenue from their work, rather than just a one-off payment. Additionally, it can help to deter unauthorized use or reproduction of the model.
However, it is important to note that asking for royalties can also potentially limit the number of people willing to purchase the model, as it increases the cost of each piece. Additionally, some people may be turned off by the idea of paying royalties.