IMHO equating commits and PRs puts undue pressure on the scope and quality of a given commit, adding potential for unnecessary stress and eliminating the benefits of an additional buffer / layer for aggregation of changes. A PR representing a sizable feature or refactor might naturally contain a dozen commits, each dedicated to a logical area or a requisite subset of the whole. Assuming on principle a goal of keeping main in a known-good state, such intermediate and incomplete changes (fine in an unstable feature branch) would wreak havoc.

It's equivalent to asking, "Why would you have more than one story in an epic (or task in a story)?".

If your PR has more than one commit, each one should be deployable in isolation. Which means you can split your giant PR into smaller ones that can be reviewed independently.