Not everyone develops and commits the same way and mandating squashing is a much simpler management task than training up everyone to commit in a similar manner.
We also do conventional commits: https://www.conventionalcommits.org/
Other than that pretty free how you write commit messages
Besides, they probably shouldn't make PR commits atomic, but do so as often as needed. It's a good way to avoid losing work. This is in tension with leaving behind clean commits, and squashing resolves it.