Games Workshop doesn't hate us! They enjoy throwing maddening puzzles at us to solve with out Immaterium-warped minds
In C:CSM it's stated that the assault on Terra and the Horus Heresy as a whole began falling apart when Horus (the chosen champion of the whole Chaos pantheon and unifying force/will) fell by the Emperor's hand. The Daemon Primarchs aren't particularly active, and even if they were, none of them would feel inclined to follow one of the others (espcially with the warpspace-realspace duality of conflict between the gods of Chaos; Tzeentch doesn't really benefit from Nurgle taking the lead). Abbadon is closest to mimicking Horus' level of all-around patronage, but even so, I haven't seen him ascend to Daemonhood yet...
So, as far as I can tell, the only Legions capable of fighting as organized groups are the Word Bearers, Iron Warriors and Alpha Legion (as well stated by Pheobus). Should Perturabo or Lorgar actually care to try, their respective Legions would follow him into (out of?) hell again and again. Against a relatively unstable Imperium, one or two Chaos Legions would crush the Cadian gate: hundreds of thousands of horribly devoted Chaos Marines don't exactly scream mercy.
As for the Alpha Legion, they've effectively been turning the Imperium against itself with all the intrigue and deception they cook up. If they ever really felt like it, I'm pretty sure they'd start picking apart the Imperium piece by piece (which they may be already doing very slowly, letting aliens, mutants, etc. save them the butcher's bill). Either that, or they lost sight of their objectives after they failed to win the Heresy: the whole point of them going rogue (according to Legion
) was to win the Heresy and bring Chaos to full power so it'd burn itself out. But they didn't win the Heresy, so now they're either fighting it slowly (Long War) or have decided it's a doomed cause and attack out of bitterness.
However, Lorgar and Perturabo are far to busy with warp-stuff now that they're no longer mortal. Funny how that works, you know, ignoring the material universe now that you can effectively cross dimensions at will.
And the random renegades from after the Heresy? They
are the space pirates. Tehy left for power, glory, and the booty, mateys, arrr *image of Huron on bow of seaship*
Edit 1: I also forgot the period Black Crusades; they do happen, they just lose steam when the Chaos warbands realize they can just go for their own goals. I wonder if Abbadon has a permanent handprint on his forehead from the number of times he's facepalmed at a failed Black Crusade...