Why does nuclear fission release so much energy?

Nuclear fission releases a significant amount of energy because the resulting fragments are more stable than the original heavy nucleus.12

This process is governed by the binding energy per nucleon. Heavy nuclei, such as uranium-235, have a lower binding energy per nucleon compared to the lighter, more stable nuclei they form upon splitting. When a heavy nucleus fissions, the total binding energy of the resulting fragments is greater than that of the original nucleus. This increase in stability means that the final products have less total mass than the initial reactants.123

According to Albert Einstein’s mass-energy equivalence relation, $E=m{c}^2$, this "missing" mass—known as the mass defect—is converted directly into a large amount of energy. In a typical fission event, this energy is released primarily as heat and kinetic energy, which can be harnessed for power generation.3124