That's a requirement that you're imposing. If nothing exists, no concept of absence of existence exists. Concepts are for humans.

How can nothingness exists, if it is supposed to not exist since it is nothingness?

Concept are not really for humans, but humans can grasp them. Or would you say that the sun only exists because (some) humans see it?

It's not because a human is unaware of something that it does not exist. Its concept is still there somewhere. Independent of its treatment by human cognition.