Let me set up a hypothetical situation here: I'm a subject matter expert. You want me to review this paper that was submitted, to see if it is up to the standards of your publication. It is a dense piece, at the edges of innovation in my field, so it is going to take some serious brain power and time for me to review the paper, assess its findings, and then provide you with my official recommendation/feedback. Ok. What are you going to pay me for this service? For my expertise? For my decades of schooling and experience that have made me a subject matter expert in my field? My Question: In the world where "information wants to be free", and it is trivial to share any piece of data with any other person in the world, who pays for the highly specialized and skilled service above? Where does that money come from?
It's volunteer work for a for-profit company. Journals don't pay scientists. Scientists pay journals to publish in them, then the journals ask someone to review it. The reward for that work is padding the CV, and that's pretty much it. It's "you get exposure", except because academia is as Taylorized as everywhere else afflicted with administrators, it is actually worth paying journals to leech of your work and volunteering to do their editing for them. They still exist because of tradition and because universities are just businesses with pretensions at this point and are run by the same asshats as every other business and these are the arbitrary hoops those asshats have decided to judge their employees' ability to jump through.