Sufficiently powerful Voidsent can't cross natural dimensional fissures and they are by nature chaotic creatures, so Voidsent have never launched a full-scale attack on the Source. (May or may not change depending on Golbez' plans.)

Voidsent adhere to the rule of power; if we're able to take what we want, we're wholly justified in so doing by their standards. I don't think they much care.