They were. Played a role in siege warfare. And then the invention of gunpowder led to end of castle walls. Magnesium / Greek fire was used, well, by the Greeks.
But these were rare rare substances used for a particular purpose. You wouldn't just run into large barrels of explosives just hanging out in caves. When I see the piles of explosives I experience the same sense of ironic detachment that I held onto while playing DOS.
In D&D if you want things to blow up you find a mage.
off (this) topic -- your point about strength being more important than intelligence for a mage is a good one.
