The WH40k wiki states that Khorne, Nurgle, and Tzeentch all came to being during Earth's middle ages - at most only a few hundred years apart from each other. The descriptions imply that it was Earth's events who marked their birth.
[Khorne] is the mightiest and the oldest of the four major Chaos Gods, fully
coming into existence in the Immaterium sometime during Terra's European Middle Ages in the early 2nd Millennium, his birth heralded by an era of wars and conflict that raged across the globe
[Tzeentch] was the second of the Chaos Gods to come to full sentience within the Warp, sometime during Old Earth's Western medieval period in the 2nd Millennium. His birth marked the maturation of human nations and politics
[Nurgle] is the third of the Chaos Gods to fully awake within the Warp, emerging during the 2nd Millennium in the midst of Terra's European Middle Ages, as great plagues swept across the world heralding his birth.
At this point in the history of WH40k, there have been multiple galaxy spanning wars that have ended entire races of godlike beings. But a few dozen million warring, diseased, plotting Europeans millions of years later were the creation of the entire Chaos pantheon at this point in the WH40K universe?
How weren't they born earlier?