Old lore was that humans are responsible for the creation and empowerement of the chaos gods except slaanesh. And while it was never mentioned for ages in franchise at least the empowerment is certainly true since pre Horus Heresy the presence of chaos in the materium was so pitiful that 200 years of Great Crusade barely managed to discover anything substantial. Hell, Word Bearers actively searched for any chaos with dozens, maybe hundreds of fleets and only found it after years of searching. Also while you look at the manifestations of chaos there's barely anything that can't be traced to humanity, there's much talk about chaotic xeno spiecies but their actual effects are always very local (except eldar and slaanesh).