That's because, as any educated person knows, you cannot "prove" that something doesn't exist!
I am agnostic by the real definition of it (i.e. gnosi =knowledge, a=not, so something cannot be known and so not be proved or disproved and the only thing there is is belief –either in his existence or none existence) in this matter. But your statement is false, there are many things that can be proven to not exist, like a flat earth or a earth cantered galaxy.
