... but often it does not get picked by other committers. As "maintainer timeout" is searchable you can see a long list of documented maintainer timeouts still open for a long time period:
Would be because it's not auto-assigned. So committers doesn't know whether or not other committer is already working on it or not, thus, don't start to work on. And also, they don't know whether the PR is still actually persists or not (as in some cases, updates by any other PR fixes it).
This is why posting to freebsd-ports ML is often helpful. If any committer having time noticed it, it could be handled.
And why sending direct email to the maintainer is not always helpful.
Not all maintainers are committers, thus, cannot commit it by themselves need committer to commit. There should be cases that a maintainer is looking for a committer for approved patch.
Another concern would be the complexities. Any too complexed-to-confirm problems/patches takes long time to read/build and test, make it delay long.
And in some limited cases, PRs are racy.
For example that recently resolved, x11/nvidia-driver* shared single distinfo, making them racy. If, for example, PRs for x11/nvidia-driver and x11/nvidia-driver-470 arose at the same time and both including distfile updates, both need to touch the exactly same file, x11/nvidia-driver/distinfo (and in some cases, both want x11/nvidia-driver/Makefile), thus, racy.
This is significantly reduced by
dividing distinfo into per-port basis.
There are still races if both needs something to do in x11/nvidia-driver/Makefile, though, it would be not so often.