How are there still CSM after ten thousand years of skirmishes?

The Thousand Suns, I get. They have the Rubicons. Those can be reused, even after being "killed."

But, how are there still Word Bearers, or any of the other Traitor Legions? Their Geneseeds have been corrupted by Chaos, and it's not like Chaos has the temperament to slowly transform teenage boys into CSM. They seem to die in every piece of literature, with no means of reproduction.

What about Titan Legions? They can't build more, can they? If that's done... where is it? Why haven't loyalists laid siege to and destroyed their fabrication hubs?