Elves can see as far as an eagle, and shoot the eyes out of a fly, if it suited their fancy. Humans invented telescopes, and microscopes, and crossbows, and cannons. And the Elves thought themselves lucky that the humans turned all that deadly enginuity towards each other, and not towards other species.
Dwarves are as tough as the stone they hew, sharper than the diamonds that they mine, with tempers hotter than the metal they smelt. But it was the humans who invented methods of detecting underground gasses, who invented means of draining water from the mines, and means of pumping clean air down into the dank depths. Many a Dwarven miner had been shocked and awed by mining into a human dig from the other side of a seam.
Only the Orcs, a race feared by many, could hold their own against the humans. Indeed, the Uruk'hai keep the humans busy with escalating methods of war. And yet, again and again, the humans beat them back through endless feats of enginuity.
Humans hold games where ancient feats of war are a competition. Who can shoot the most accurately. Who can run the fastest. Who can throw a spear the furthest. And who can throw a cannon-ball the furthest. With nothing more than their bare hands and muscle power.
And they did this for bragging rights.
When the humans were not making war with the Orcs, they made war on each other. If one group found a new way of committing horror on another, the others would adapt it like wildfire.
Humans. You couldn't just leave them bide for a century or so. Their short lives made progress a desperate race to make a mark, to be remembered. If only for a decade or so.
They were persistent, too. If a disaster occurred in a new settlement, they would return, analyse, and rebuilt stronger, with measures in place to warn of another disaster. And measures in place to protect the people.
About the only thing that kept them in check was the diseases that assaulted them, that had origins in their own feculence. Not that anyone could tell them that. Humans believed that plague came from smells.
Those who knew the truth surrendered to allowing the humans' beliefs very quickly. If the humans ever learned about the power of sanitation, they would overrun all of the realms. They would end the Dragons, they would end the Orcs, and they would, eventually, end the Elves and the Dwarves.
There is a saying, in the magical realms. "Best to leave humans as their own worst enemy." And they don't say it around humans.
The Elves intervene, occasionally. Making certain that the humans survive their own disasters. The humans are worth keeping around, they say. If ever a worse threat rises, the humans are bound to knock it down. And cheer whilst they do so.