Because nothing is something. Nothing is an illusion of the mind because if a space so much has an atom in it it has something in it and even if you somehow removed every atom in the space you identified the spaces as something by calling it nothing.
And if someone argued that nothing is something that doesn't exist, something that doesn't exist is still something, it's nothing. And nothing is something even if its anything.
I'm sure you've never seen an empty glass either.... No matter if there's water (or other liquid) in it or not, it can only be truly empty when there's no air in it either.