Ontology isn't just about what is "material"
"Ontology ultimately concerns the meaning of existence. What are we actually claiming when we say, for example, that there are meerkats? Many people believe that this question is addressed to physicists or, more generally, to natural scientists. In the end, everything that exists could just be material. After all, we don’t seriously believe in ghosts, which can arbitrarily violate natural laws and unrecognisably whirl around us. (Well, most of us don’t believe this.) However, if for this reason we claim that only that which can be investigated by natural science exists and can be dissected, or pictured, by means of the scalpel, microscope, or brain scanner, we would have missed the mark by a long shot. For in this case the federal state of Germany would not exist, nor would the future, numbers, or my dreams. But, because they do, we justifiably hesitate to entrust the question of Being to physicists"
Markus Gabriel, Why the World Does Not Exist (Cambridge: Polity Press, 2015) pp. 7-8