They don't. But reviewing individual commits in the GitHub UI is hard.

A set of stacked PRs is exactly the same as a line of commits. The only difference is the UI, but the UI is the important bit here because lack of UI is what's stopping folk from doing that today.

Even when I've developed my changes as a stack of commits, I'll feed them to my team one commit (and one PR) at a time so they're easier to review — and I discovered that GitHub had turned on stacked commits UI because for one particular project I'd manually created a set of PRs in advance (with the right bases) and GitHub offered to create a stack out of them.