Because it's shared. Assuming you accept the existence of other minds, what makes reality real, is that it's shared. Schizophrenic hallucinations are no less real to the sufferer than so-called normal experiences. What makes reality real is that every shares the experience.
That raises interesting questions about what is real and what is not, but also questions about how much of reality is really the residue of culture and worldview.
That's where science and experimentation comes in to some extent - individuals should be able to reproduce results of someone else and see the behavior of reality for themselves.
That raises interesting questions about what is real and what is not, but also questions about how much of reality is really the residue of culture and worldview.