Why don't honeybees sting hornets for defense when their hive is attacked? It seems like a good weapon against bigger animals - why not against hornets?
Smithsonian mag says that the bee stings can't penetrate through the predator hornet's armor, which is simply a case of the hornet's armor being tougher than the blunt barb of the bee sting. https://www.smithsonianmag.com/smart-news/invasion-murder-hornets-180974809/
To examine their behaviors of penetrating into different materials, we performed penetration–extraction tests and slow motion analyses of their insertion process. In comparison, the barbed stings of honey bees are relatively difficult to be withdrawn from fibrous tissues (e.g. skin), while the removal of paper wasp stings is easier due to their different structures and insertion skills. The similarities and differences of the two kinds of stings are summarized on the basis of the experiments and observations.
Some research suggests that bee stings have evolved to ward off vertebrates, physically and chemically. Wasps have more slender stings and perhaps that's why the hornets prefer bees nests.
Actually, a lot of insects have phenomenally hard keratin, beetles and the bulldog raspy cricket, as far as i know, even parasitoid wasps which have specialized stings evolved to cope specificially with insects are more common on caterpillars and spiders than strongly armored animals like big beetles.