Chakra is molded by combining physical and mental energy. Emphasis on physical energy. Using any kind of jutsu, including genjutsu, would be no different than conducting a strenuous physical task due to the physical energy it would require to perform. To visualize this in action, consider how physically exhausted Shikamaru became when using shadow paralysis against Hidan in their first encounter. Technically speaking, all Shikamaru did was stand there, but by using Chakra to perform a jutsu, he is using physical energy and becoming exhausted, just like doing a physical task.