I suppose, they are being more reasonable now, it must take them a few millenia to stop being groggy from a sixty million year nap.... it'd take me twice as long.
From the codex on Chaos Space Marines is the best description of why the Chaos Marines abandoned the Imperium- there was no reward. For Astartes, the Imperium demands eternal servitude with absolutely no rewards besides more servitude and a fancy gravestone. Meanwhile, after subtle manipulation to gain the eye of Astartes, Chaos offers potentially anything. It emancipates Astartes from their life of eternal servitude and offers anything from being a psychopathic killer to being the lord and master over your own whole damn planet, even a sector, and rule as you wish so long as you please your respective god(s). Be it a life of a pirate lord, or that of a Chaos Lord forging his own private empire to rule, it's certainly better than the normal life of an Astartes and offers a chance to ascend to minor godhood if you truly please your new lords and masters. (And hell, even then, you stand a chance of being a revived if you gain notoriety, multiple times.) As for humans, there's two main types of cultists. Those that are utterly mindfucked by the Chaos Gods and turned into little more than unwitting pawns, and those as well that seek to rebel against the Imperium. Within the group that maintains their free-will, there is another set of two groups. One, much like Abaddon and most of the Black Legion, seek to succeed where Horus failed and overthrown the Emperor and build a new Imperium of Mankind that worships the Chaos Gods- similar to the civilization that birthed the Word Bearers (which I might also add, was stable). Then there are those that simply seek service of the Dark Gods and elevation by them in their service. Which is a preferable life, considering the average life of a citizen is dealing with 90% tax rates, serfdom, and likely being killed by the numerous gangs that wreak havoc on Hive Cities. The only place that's likely a decent location to live as a human citizen in W40K is either Terra itself or Ultramar (as the Ultramarines are a pretty nice Chapter to live near a bog standard human, say, over the Minotaurs or the Blood Ravens). Also, anyone who claims the GEOM isn't a god is a deluded idiot that hasn't read any of the Black Library material, or especially the Horus Heresy series. Not only is the GEOM a warp deity incarnated as a human, he's a perpetual. He isn't ever going anywhere. As for the Necrons, they lack means to destroy the Immaterium. Rather, their intentions are to seal off the Immaterium from the Materium permanently, and prevent Chaos from accessing the W40K Milky Way, potentially its whole universe and forcing the Chaos Gods to go elsewhere. Then after exterminating everything, they switch back to organic bodies.
Then the correct answer is, if chaos is like the music industry? is then only a part of it but not entirely.
Why do people willingly steal, lie, cheat? We do things because we're emotional and we each have trials and regrets and challenges. None of us can see into the future. Life is really painfully hard for most people, even a moment of help at the right time can mean all the difference. Nobody joins chaos, chaos offers help and some people accept it.
Let's be honest. The only real reason someone would actually worship the Chaos gods in the story world is probably because, 1. Secrets of the universe, 2. Anger and rage for feelings of betrayal from mankind, 3. They're broken and have no hope, 4. They believe there is something severely wrong with the Imperium and want to learn what it is, 5. No ability to have a lover or have any human interaction whatsoever. No sex would make anyone angry.
