It would kinda go against the idea that meat bags are weaker, and even the general design of the Driven Assimilators. I don't think robots would consider that organix reproduction is better than building robots, especially since you'd need to cybernetize all newborns. It maybe also break balance, since it means you would basically be a Machine Empire with cybernetic organic pops without happiness.
And there's also that in science-fiction, the example I know of Driven Assimilators don't do that. They don't have any reason to let the assimilated continue their life, instead, they will take everything seems good for them and use that. They are assimilators, their goal isn't to breed other pops. They may use the assimilated species to breed themselves, but nothing more than that. We have to understand that there's actually a difference between what the player may think is the best strategy - aka more pops - and what the AI Overmind may actually think. And for him, organix reproduction is clearly inferior.
