The prime reason why humans cannot produce their own energy other than from eating food isbecause of the anatomical differences that humans and other animals have in comparison to the plants. Humans tend to get their energy by breaking down the food that they eat.
The plants on the other hand make their own energy using sunlight, carbon-dioxide, and other nutrients from the soil. Hence, comparing the two would not make enough sense because both are formed differently and have been developed differently by nature. Other ways of making energy might be developed as research does goes on into this topic.