Unfortunately, that's not the 90% case that I see. Most of the time a multi-commit PR contains N-1 commits of incremental development and one final one that fixes all the tests and typos and removes debugging print statements. Neither the project nor the author benefit from having those intermediate commits integrated verbatim.
I know, it might be too much to ask of people... oh well.
Just because you smash away at code for hours doesn't mean that's how your commit history should look. You can revise history in ways that are beneficial instead of destructive.
Sure they benefit. You can see the thought process that went into the commit. If there's something that seems weird or out of place you can see how it evolved into existence. That can be exceptionally useful.
This is too myopic and explains how you can end up with good code, but bad architecture. A good review ensures both.
I like my PRs to be about high level goals, and I want them made of lots of commits I can review. The commits themselves can be the result of re-basing (which is fine within feature branches) and maybe not truly chronological.
If that were true, the optimal solution is PRs with individual commits that all pass testing. I find it much easier to review a series of small changes for logical correctness than mashing them together into a single PR. Github recently added this as a feature, so I'm not in a completely invisible minority there.
And then, when the review is over, having discreet commits makes git bisecting down to the commit that broke the system more granular.