The Orks' ancestors in ancient times had a name: the Kolt Orks.
They were war machines painstakingly crafted by the Old Ones. They knew neither fatigue nor fear; every aspect of their creation existed for war.
The Orks' exceedingly simple growth cycle
Although the Kolt Orks were not the most powerful weapons, they possessed neither the terrifying technology of the Necrontyr empire nor the psychic gifts of the Aeldari empire.
But they were tireless, terrifying enemies capable of enduring any losses. The Necrontyr empire went into slumber rather than endure prolonged warfare, while the Aeldari chose to ignore them.
And so the Kolt Orks found themselves with no wars to fight. Even a precious blade that could cut through iron like butter would rust after lying dormant too long, let alone the Kolt Orks, whose very purpose in life was war.
Orks during the War in Heaven
After the powerful C'tan vanished and the Necrons, overlords of the Milky Way, entered their slumber, the Kolt Orks descended into meaningless civil wars. Their strongest individuals gradually died in the infighting, leaving only a mob of fools.
Perhaps, after another ten million years, the weapons of war created by the Old Ones would lose their last vestiges of reason and become nothing more than strange, stupid fungal creatures.
But just then, an unexpected race stepped onto the stage of the Milky Way.
Humanity.
A perplexing and fearsome race, they reached the stars on their own in an extraordinarily short time, then became one of the Milky Way's dominant powers.
Like a crowd of curious children, humanity kicked open the doors of the Milky Way and met the Orks' fists with well-meaning smiles. The moment the two sides encountered one another, they plunged into a protracted war.
And this, in turn, gave the