The people who are maintaining the repository likely have to organise meetings with legal and business teams. When no agreement can be reached, doing nothing is the easiest way forward. They have to do this next to their normal work.
Sending out DCMAs is a different process done, likely done by the legal team.
The PR is not in their repository, it's in the fork. Pull Request refers to "pull this from my repository, here's a link", GitHub just presents it in a convenient interface.
(But you're right, those who send DMCAs are likely to just a send a link to the original repository :)
One can assuredly delete the pull request from your own list of open pull requests, right? It isn't that the information exists in theory on GitHub: it is that it is still listed right there when you go to check the health of the project's open issues/PRs.
