I think that we have a perception bias towards things that interest us.
Since the elements in the top-right corner (C, O, N, P, S, Si, Fe, Al, Na, …) are interesting to us, that’s what we typically look at. And in that region, things are fairly balanced. It’s only in the regions where we don’t typically look, where we said “let’s just make it all metal so the categorization is done, call it a day, and move on”. I think.