If you squash your prs there is no point in spending time on making individual commits nice and isolated or their messages as they'll be aggregated into one anyway.
When you do large PRs you want to checkpoint your work and commits have less meaningful descriptions, it's just next step of steps that you couldn't plan ahead, they're popping up as you go.
The fact that you can do that indicates that they could be simply separate PRs merged to main branch independently instead. It's usually desirable as it avoids longer living PR braches that tend to create conflicts and are harder to review.
It is a good point. In many cases, like solo or small teams the conflicts are not a big problem, but I see what you are saying. I have also been in teams that used feature flags more heavily, and it is a possible solution, too.
When you do large PRs you want to checkpoint your work and commits have less meaningful descriptions, it's just next step of steps that you couldn't plan ahead, they're popping up as you go.