FWIW, my git is configured in such a way that pulling from github also pulls all PRs for that repo, so, in effect, all PRs are applied locally, and I can review them however I want.
I do think that email threads (when all parties are disciplined and have them properly configured) are superior to the PR+comments format for discussion, but applying patches from a mailbox has never seemed to me like a pretty and reliable way to go, so at least github helps with that.
At any rate, git sucks for reviews in general because it still lacks mercurial's "mutable-history"/evolve approach to safe and distributed history rewriting (or "diffs of diffs"), and that, to me, is saddening as one more evidence that git's monopoly is causing stagnation and unnecessary pain in this space.