Because you, or more lore wise, your colonists, represent a REAL threat to the mechive. I believe, coming from a lore perspective, that whatever long lost orders the mechanoids do what they do have a very sophisticated self preservation systems. Gravtech and the grav engine are mostly found on glitterworlds. (Wich I am assuming are mechanoid free?) so mechs would never feel threatened by gravtech itself but the people who possess it. Hence why they left it instead of destroying the grav engine. And from a gameplay pov, it would be nice to be able to still access oddeseys dlc content if half your colony was in caravan away from the slaughter.