Why do saunas have to be made of wood?

KLAFS FAQ for well-being

To create a pleasant, mild and salubrious atmosphere in the sauna, most of the interior needs to be manufactured from wood. Expansive glass fronts and stone walls may be contemporary and aesthetically pleasing, but the more wood, the better the climate in the sauna. Because only wood is capable of absorbing heat evenly during the warming-up phase, and later emitting it again evenly. Wood is also capable of absorbing the increased moisture in the air when water is poured over the hot stones, keeping the humidity level inside the sauna below 10% even when the temperature of the air is 90°C. Another major advantage of wood is that it does not heat up through thermal conductivity to the same extent as other materials, so sauna-goers don’t get burned.