Sure, if the publisher actually provides that. They don’t. (Or rather, they farm it out to volunteer labor while collecting a fee anyway.)

Nature journals actually do provide professional copyediting services for all articles accepted for publication. For commentaries (from my own experience), the editing process at Nature is quite intensive.

That just pushes out the argument by one level of indirection. The publishers 'curate and edit' the board of editors for their journals. In turn, these boards of editors curate and edit the articles that go into the journal.

Nope, they don’t. The “volunteer” reviewers are drawn from past published authors, who in turn had their articles selected by previously published reviewers. Turtles all the way down.

This is not true. Reviewers may or may not have been past authors. I have reviewed for many journals in which I have never published.

Conferences can fill that role instead. Their business model doesn't require paywalls or the copyright police.

All the conferences in my field have literal security guards checking the badges that are only handed out after paying hundreds of dollars to register.

Of course, but you don't need to attend the conference to read and benefit from the research, and that's the important thing.

The journal business model usually requires restricting access to the research, while the conference business model still works when the research is free.

Except that conference proceedings can be exactly like a journal, i.e. you often need to pay to access them.

There must be some benefit to attending conferences, otherwise why would scientists attend them?

Yeah, but it's usually more about a chance to socialize with other people in the field.

It's also interesting to hear other people's talks that serve as a "teaser" for what's in the paper.

That's the whole point. Conferences can provide a benefit worth paying for without putting the research itself behind paywalls like journals do.