This isn't really true. I work for a non profit scholarly society publisher, and we pay millions of dollars a year for copy-editing/formatting and also the work to find referees that do not have conflicts of interest/are not busy/actually respond and are qualified to assess the paper is indeed costly. If it were zero or negative sum there'd be loads of open platforms out there for peer-review and publishing.

I am also a big fan of open-access and hate paywalls myself. I also don't like big, for profit, predatory journals. There is value in curated and managed journals. Just not the value that these big corporations are reaping from them.