Why is the Earth tectonically active and not Mars or Venus?

Mars is, evidently, too small, and since it doesn't have a magnetic field, it's core must be solid and not fluid. Perhaps it simply cooled off too quickly to have a crust thin enough to form plates. As for Venus, it also doesn't have a liquid core since it has no magnetic field. Some astronomers think that its crust is so thick that it can not break into plates that move. But that every 750 million years or so, heat builds up from decaying radioisotopes inside causing a catastrophic melting of the core and the churning over of the crust like oatmeal in a pot. No feature is seen anywhere to be older than 750 million years unlike the Earth.

Frankly, we will not know what is going on inside these planets until we can plant seismographic stations on their surfaces like we did with the Moon.