It comes as no surprise that when Orks first witnessed an emperor titan they felt truly humbled and afraid, an alien emotion for orks. They heard the humans refer to it as "the emperor" that vague, never seen warboss of all humanity that all pink skins followed but was never seen. For orks, this was a foreign concept. A leader always led from the front, revelling in the carnage where the fighting was thickest. The strongest, largest ork was always the boss, always proving their strength in battle. When they first saw an emperor titan they realized a warboss that huge, that powerful, with that scale of firepower, it was no wonder humans everywhere obeyed even if it wasn't around.
Before too long, the first gargants began to roll out of mek workshops as orks constructed effigies of their own gods, just as the humans had.
https://www.youtube.com/watch?v=_Vu2nlAZm1U 