According to Starcraft lore, the Zerg descended on the human colonies because the Overmind observed the potential for psychic abilities amongst the humans in order to help them fight against the Protoss. In the game, you can infest Terran buildings and create Infested Terrans. If this is done so easily, why didn't the Zerg just capture some Protoss specimens themselves and incorporate their genetic information instead? They already had developed far superior psionic powers.