This question already has an answer here:
Most of the work involved in producing a journal is done by people paid at rates higher than most academics, but the those with the rarest skills, those able to review specific scientific articles, provide their labor for free. In my fairly limited experience most academics are over worked, under paid and not particularly positive about the closed access journal system or shareholding capitalists. Most reviews are conducted anonymously, so reviewers do not receive direct credit from the scientific community. Funders (in my limited experience of the MRC at least) care about papers a group author not those they review. The big journals at least are profitable companies that pay dividends to their shareholders, and I presume that if they were unable to get this work done for free they would pay for it, making the academics better off and the shareholders receive less dividends.
Why do so many academics provide this service for free?
I saw this question, but it does not seem address the economic imbalance or the supposition that if no one did it for free then everyone would get paid.