It's not that publishers do little (there's a lot that you don't usually see, but it's not the peer review), just that not all of it is that necessary, and that it still pales in comparison to the prices they charge.
Look at arxiv.org, that papers there are made purely by academics from start to finish and are already publication grade (if not better).
(That said, typesetting and writing are more and more often offloaded to academics, but there's also things like e.g. making HTML versions and system-readable versions [1] available, which is a cumbersome manual process.)
[1] https://en.wikipedia.org/wiki/Protocol_for_Metadata_Harvesti...
So yes, peer review is useful, but it also often happens after publication at arXiv, outside of the journal submission.