Why can't real-ness just be functionality? People often resist this concept, but it seems sensible to me.

Exploring the function of things, in fact, how we know about the universe - when we talk about what something is, we'll really talking about an aggregate of functions that it has (e.g. we know that if we do something to a part of the universe, something will happen - since the result varies by the part of the universe we're looking at and the conditions under which we perturb it, we can divide the universe into "things.") We can say that atoms (for example) are real because we have observed consequences of our actions that (as far as we can tell) could only happen if something fitting our description of an atom existed.

Thinking of something as "real" in the sense that it seems like a sensible and somewhat self-explanatory entity is just a matter of familiarity, I think. We think of atoms as "real" because we've grown up conceiving of the world in that way (which is only the case, one would hope, because they have been shown to be "real" in the more strict scientific sense I mentioned above.)