Because if we were made of antimatter, then that's what we would have declared to be matter, and it would still be the case that the stuff we're not made of would be the antimatter.
Both questions are valid. Antimatter is an arbitrary label, but it is a specific thing with properties that prevent it from dominating the universe. It's not a coin flip with the victor earning the title of matter, it's an inevitable outcome that scientists are trying to find the mechanism for.