Loyalist Marines only rarely go traitor. Likewise, there are very, very few CSM's that have survived since the Heresy, and most of them are Lords/Daemon Princes in one form or another.
The vast majority of CSM's are recruited just the same as loyalist Marines, are trained by their legion or warband, and equipped by forgeworlds controlled by chaos forces.
The reason CSM's have inferior equipment is because they don't control forgeworlds capable of producing, say, storm bolters. They might get a handful from new traitors, but not enough for there to be a significant number of storm bolters. Thus, they still use combi-bolters.