Not all planets have stopped erupting. Io, a moon of Jupiter, I think still erupts molten ice (in other words, water!). But Mars and Venus, which both used be more like the Earth in that they had volcanoes (I know for sure that Mars did anyway), are now volcanically dead. I don't have a good answer for this for Venus, but the volcanoes on Mars are probably dead because Mars is smaller than the Earth and has lost most of its heat already.
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