Who selects the reviewers for a PubPeer review?
The problem with PubPeer is that it is (as I understand it) a commentation system for people who have something to say about an article. But that's not what we need review for. What we need review for is to provide context for people who are not in the field.
A good review is a lot of work. It needs to be from a separate group (so as to avoid conflict of interest). It is not supposed to be self-serving (so it shouldn't be because someone feels they need to add to the discussion). Instead, a proper review is that someone asks a qualified expert to provide context and evaluation of a product.
The first key (conflict of interest) is why we can't just leave preprints unreviewed. The second is why we can't just leave it open for people who are helpful. The third is why editors ask reviewers to provide reviews, and why we don't want authors to be the ones soliciting the reviews.