Appendices to other people's papers - what's the incentive?
1) The results of an appendix are usually much less significant than the results of the main paper.  It usually would not be within the cultural norms to produce a joint paper, because the authors of the appendix did not actually contribute to the important parts of the paper; in mathematics, a 3%-5% contribution doesn't usually warrant adding an author to a 2 or 3 author paper.  It would be possible to publish the appendix separately, but only in a bottom-tier journal that accepts any contribution that is correct and not plagiarized (or at least a much less well-regarded journal), at which point the appendix may very well get more attention attached to a somewhat important paper rather than buried in some obscure journal.  Also, producing a separate paper is more work, because the authors would need to write an introduction, set notation, restate the results of the main paper, and so on.

2) Only idiots care about these counts.  If an insignificant contribution of the kind you would put in an appendix actually matters for your career, you're doing quite poorly as a research mathematician.  (Exception: sometimes a top mathematician will write an appendix whose contents would be considered significant if produced by me but are still rather insignificant in the context of their work.)  If your university gives you a raise or a bonus per paper, they are idiots, and you should discuss with your dean beforehand how an appendix will be treated.