The heat is spread out to a much larger volume.

Heat engines are most efficient when the temperature difference between the hot and cold side is high, so you need to keep it that way to extract energy.

A nuclear power plant achieves this by converting extreme heat from a small, but very angry rock to a huge lake of slightly warmer water.

There exist reactor designs which operate at higher temperatures, thus increasing efficiency, but they're complicated as everything needs to be more heat-resistant.