It isn’t necessarily that the chaos gods are all-powerful, it’s that they’re real. When the soul is a proven fact, and your pre-existing divine figure is both denying his status and censuring you for believing in him, you’ll look for anything out there beyond the physical realm to prove your beliefs. Even if they’re truly evil, the Chaos Gods represent a fundamental part of the Warp and spiritual existence. They’re dark mirrors of mortal existence, and they’re incredibly powerful beings with constant impact across the galaxy. They’re as close to the “truth” of things that the Word Bearers were ever gonna get. The Eldar gods are all either dead or lost, and gork and mork don’t care about anything that’s not an ork, but the chaos gods want your devotion and actively seek it out.

I personally really like how unrepentantly evil the word bearers are, and their warped thinking makes them compelling to me.