Why can’t chemical reactions serve as a source of energy for the sun?

If the Sun were entirely composed of coal, then the energy of burning this coal would be enough to maintain the current solar radiation for about 5,000 years. But the Sun has been shining for many billions of years. If coal were burned in the “solar oven”, it would have gone out long ago. Other chemical processes differ little from combustion: they also do not provide enough energy to provide radiation from the sun.