Innovating your own wheels to sell carts requires capital investment, time, effort, and risk. The saying is a rehash of common advice to focus on core competencies and not overextend.

If I sold, or even manufactured carts, I would not be eager to enter the enormous, competitive, highly commoditized, and mature wheel market. It sounds like a real battle to get production costs even on par with wholesale, lacking all the economies of scale, experience, and supply chain agreements the major manufacturers have.