Editors can act as filters, which is required when dealing with an excess of information streaming in. Just like you follow celebrities on social media or you follow pseudo-forums like this one, you get a service of information filtration which increases the concentration of useful knowledge.

In the early days of modern science, the rate of publications was small, make it easier to “digest” entire fields even if there’s self-publishing. The number of published papers grows exponentially, as does the number of journals. https://www.researchgate.net/publication/333487946_Over-optimization_of_academic_publishing_metrics_Observing_Goodhart’s_Law_in_action/figures

Just like with these forums, the need for moderators (editors, reviewers) grows with the number of users who add content.