Stacked PRs tend to encourage a series of well-organized commits, because you review each commit separately, rather than together.
What they do that the single branch cannot is things like "have a disjoint set of reviewers where some people only review some commits", and that property is exactly why it encourages more well-organized commits, because you are reviewing them individually, rather than as a massive whole.
They also encourage amending existing commits rather than throwing fixup commits onto the end of a branch, which makes the original commit better rather than splitting it into multiple that aren't semantically useful on their own.
I think the point the GP was trying to make is that the GitHub UI ought to be able to allow you to submit a branch with multiple well-organized commits and review each commit separately with its own PR. The curation of the commits that you'd do for stacked PRs could just as easily be done with commits on a single branch; some of us don't just toss random WIP and fixup commits on a branch and leave it to GitHub to squash at the end. I.e., it's the GitHub UI rather than Git that has been lacking.
(FWIW, I'm dealing with this sort of thing at work right now - working on a complex branch, rewriting history to keep it as a sequence of clean testable and reviewable commits, with a plan to split them out to individual PRs when I finish.)
Stacked PRs tend to encourage a series of well-organized commits, because you review each commit separately, rather than together.
What they do that the single branch cannot is things like "have a disjoint set of reviewers where some people only review some commits", and that property is exactly why it encourages more well-organized commits, because you are reviewing them individually, rather than as a massive whole.
They also encourage amending existing commits rather than throwing fixup commits onto the end of a branch, which makes the original commit better rather than splitting it into multiple that aren't semantically useful on their own.
I think the point the GP was trying to make is that the GitHub UI ought to be able to allow you to submit a branch with multiple well-organized commits and review each commit separately with its own PR. The curation of the commits that you'd do for stacked PRs could just as easily be done with commits on a single branch; some of us don't just toss random WIP and fixup commits on a branch and leave it to GitHub to squash at the end. I.e., it's the GitHub UI rather than Git that has been lacking.
(FWIW, I'm dealing with this sort of thing at work right now - working on a complex branch, rewriting history to keep it as a sequence of clean testable and reviewable commits, with a plan to split them out to individual PRs when I finish.)