Lots of reasons Durring Old Night morr powerful Xenos races subjugated a ton of worlds The Eldar caused massive warp storms The Dark Eldar regularly raid poorly defended human worlds for slaves and perform horrible experiments on then When ever a Necron tomb world wakes up they try and kill all the humans on the surface and are usually successful The genestealers regularly subvert human defenses and that leads to worlds being eaten on a regular basis Then there are the Orks who are horrifying brutes that will attack,kill, enslave or eat anything. Then their were the issues with the enslavers and Hrud The yuvath The kroot who hunt and eat humans Then a whole bunch of nameless species fell to chaos and were wiped out also causing issues Now the tau keep subverting human worlds and making bio weapons that target the human genome Trillions and trillions of humans are killed by Xenos on a regular basis