They definitely are used in some circumstances. For example, a sauna uses heated rocks. This allows you to control the humidity separately and not have the wate

They definitely are used in some circumstances. For example, a sauna uses heated rocks. This allows you to control the humidity separately and not have the water evaporate. I guess it would work just as well with sealed metal tanks of water, but rocks are cheaper and more traditionally available. And they can't leak. So I'm not doubting you, just wondering why an intermediary heat storage medium is preferred in practice. Maybe for some of the same reasons as the sauna.