Reviewers don't generally get paid.
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.