Different assumptions

Assuming the earth has a molten core is important, for theories from continental shift to the generation of the earth's magnetic field. There's a thought that with no molten core, the earth's magnetic field would "die", and the earth's atmosphere would be stripped by cosmic radiation, as it may have been on Mars.

For those theories relying on a molten core, the heat is a leftover, from the earth's formation, and cooling is taking place at about 100 degrees every billion years.

However, some vulcanologists believe that the molten core is self-perpetuating. For them, the source of the heat is radioactive decay.

This doesn't easily account for the coolness of Mars & the moon. They are smaller than the earth, and so they would cool more quickly. But their proportion of radioactive material is probably similar to the earth's.

This same criticism could be applied to any theory where volcanic heat is generated by some action of gravity against the earth's structure. Mars & the moon would have this process in proportion. And yet, they have no volcanic activity.