Nomads are more or less organic machines created by the daam k'vosh to safeguard sirius. They arent programmed to 'expand', and as long as key infrastructure like the dyson sphere isnt being threatened they have no interest in humanity. The nomads view humans as just an insignificant lifeform, like we view ants - you dont go trying to capture every antmound you see, do you? You only deal with them when they start getting into your house.