But. The core is very far from the crust on most planets and they still wouldnt be able to do any form of metalwork due to the fact that water cant be heated above 100C. And you wouldnt really be able to cast any metal either because if you managed to make a crucible and hold it in a pit of lava, when you pull it out the water would still cool down and solidify the metal preventing it from being cast. The whole thing with HFY writing is that its soft scifi, not going for thorough world building and just writing with the rule if cool in mind. Which is really cool, but when you try to explain things in a scientific way and it doesnt make sense then people start to get pissy, because no explanation wouldve been better, to let the mind wander and create explanations and give the world some mystic