Well... in order for Voidsent to even get to the Source, they need a portal big enough to let them through. Usually those portals are created on purpose by mages. The most powerful Voidsent (like the Cloud of Darkness) can never come through to the Source because they are too big for a portal to ever be made that they can go though.

It also begs the question of how the really powerful Voidsent can get aether if they never leave the Void... even though they by definition have to have the most aether of any Voidsent...