Our planet Earth is about as hot inside as Venus. The heat here on Earth is pretty much locked deep down bellow kilometers of rock. After billions of years, it’s still there, trapped inside the planet, escaping only slowly.
No wonder, since a thick stone wall is a good insulator and a thicker wall is even better. If you put a thick enough wall between a freezer and an oven there will be virtually no heat transfer between them.
On Venus however, the atmosphere does the locking and prevents the volcanic heat to escape. Carbon dioxide is twice as good an insulator as air is. And there is almost 100 times the amount of that insulator on Venus, than there is air on Earth. Insulating job performed well!
You may ask, but what about vertical currents of gasses? Don’t they transport the heat from the surface to the high atmosphere where it can bee cooled?
They can’t. There is only 1 cool molecule of carbon dioxide 50 kilometers high, per 100 hot molecules on the Venus surface. Gravity prevents almost all of those molecule to rise and cool. The barometric equation holds, and there can be no significant vertical exchange due to reasons of simple arithmetic. A molecule on the surface isn’t fast enough to go this high and cool itself.
The pressure 50 kilometers high above the surface of Venus is about as high as at the see level here on Earth. There is still a considerable amount of insulating gasses above 50 kilometers, for carbon dioxide is a better insulator than air.
The primordial heat of Venus is confined on its so called surface. The fairytale of greenhouse effect is just that — a fairytale.
Would you attribute the heat in deep coal mines to the greenhouse effect of the rocks above? Those rocks do the same job as the thick atmosphere of Venus. Several kilometers of rocks or several tens of kilometers of the said gas, do approximately the same.