I think that they make things out of wood because there is plenty of it, and they are only actually in danger if there is an eruption, which the dragon doesn't allow. But their main village is carved inside the mountain... I don't really know.
A young child is running around with an arsenal of heavy accessories fighting flaming bats; carnivorous birds; goblins; giant, flame-retardant centipedes; and giant, bomb-carrying rats in the middle of a volcano, and you're worried about how wooden ladders and bridges can be in close proximity to a little magma and not get burned? You have obviously never played a Zelda game before.