Why are chemical burning and gravitational contraction inadequate power sources for long-lived stars?

They release far too little energy over the timescale required for stellar longevity.

Stars require immense and sustained energy generation to maintain their structure over billions of years, which simpler sources cannot provide. Chemical reactions, like combustion, release energy based on electron shell rearrangement, which is negligible compared to nuclear energies. Gravitational contraction releases energy as the star collapses under its own weight, but this process is relatively fast, powering only the very earliest evolutionary stages or perhaps a few million years for the most massive stars. The sheer duration of a star's life, especially for objects like the Sun, demands the continuous, massive energy release enabled by converting mass directly into energy according to Einstein's $E=mc^2$, achievable only through nuclear processes.