The Thousand Sons Legion isn`t made out just of the Rubic Marines (which are basicly golems - armor animated by a soul), they also have a high concenctration of cultists, psykers, sorcerors (how the hell do you spell that?) or whatever you want to call them
Don`t forget they`re serving Tzeench
So... as durable Rubic Marines may be, the sorcs are still squeeshy, and they need to be replaced
They replaces them with recruits form tzeenchian cults
