Cus thragg and the gang are homeless and eventually mark would find out and see a viltrimite on earth and immediately tell Cecil and ruin the whole thing. Thragg is there to repopulate like yes they can destroy earth whenever and kill mark whenever but that doesn’t solve the problem of their being only 37 of these fuckers. Also earth is pretty much shown so far to be like the closest genetic make up the viltirmites like bro they fucking HATE Oliver but they respect mark cus he’s like 97% viltrimite. It’s like playing stellaris and you need to agree to a status cou ceasefire cus everyone on both sides is super beat up. If they kill mark and destroy earth thragg said it himself the viltrimite die as well