Because Matt Ward won't let it fall... In all seriousness, though, do you know how many Humans there are capable of going toe to toe with Greater Daemons? Ciaphus Cain killed a Daemon Prince of Slaanesh, Kaldor Drago beat the shit out of a Daemon Prince Primarch, Njal Stormcaller turned a Great Daemon into meat-confetti using only his mind, plus, there's evidence to suggest that the Alpha Legions are in fact a sleeper cell, ready to turn on the Chaos Gods when they are at their most vulnerable.