I wonder if part of the reason for uncited papers being so low is that authors cite their own papers in subsequent papers. For example, an author is studying field X, and publishes papers. No one cites the first paper. The author then writes a new paper on X, but cites the first paper. Thus, although no one but the author cited the papers that the author wrote, all but the last one have citations in other papers.

Another way this works is that authors in the same department may cite each other's papers especially if the fields are somewhat related.

I wonder what the data would look like if they excluded citations by the authors of the original paper.