The reason is tradition.
Because they got money in turn for publishing and distributing the books in the past, now they want to continue getting exorbitant fees even though they are not providing any real value any-more.
The most important aspect is peer review. At least in physics, journals assign your paper to an Editor (a scientist), that may reject it directly if it is not scientific. If it is, they will send it to another scientist to read the work and (a) suggest rejection, (b) suggest accepting the work directly or (c) in the most common scenario accept the paper for publication after some revisions. The editor reads the review and the informs the author of the paper accordingly, and the story iterates until the work is fine for the reviewer.
There can be more than one reviewer (a.k.a. referee). The editor is what the journal offers, together with some spell checking service before publication. Editors are payed, and referees only sometimes.
In my (perhaps more limited) experience, the editor isn't an expert in the field, they're just the person who finds the volunteer reviewers who are the experts. Sometimes they find expert "guest editors" who are volunteers. Also, the final formatting / line-editing was outsourced to India.
Academic publishing is a scam. Don't volunteer for scams -- only review for open access journals / conferences.
They can do that without a publisher though. My partner reviews papers all the time, and she would continue to do so even if this ridiculous ponzi scheme didn't exist.
it's not as if peer review is some exclusive thing for scientific papers anyways, any open source technology has it as a matter of course (provided it's reasonably popular).
Just look at 3d printers, that technology is almost entirely created by hobbyists who just looked at each others' work, shared what they think works and doesn't work, and make improvements based on that.