Elsevier pays its reviewers very well! In fact, in exchange for my last review, I received a free month of ScienceDirect and Scopus…
… Which my institution already pays for. Honestly it’s almost more insulting than getting nothing.
I try to provide thorough reviews for about twice as many articles as I publish in an effort to sort of repay the scientific community for taking the time to review my own articles, but in academia reviewing is rewarded far less than publishing. Paid reviews sound good but I’d be concerned that some would abuse this system for easy cash and review quality would decrease (not that it helped in this case). If full open access publishing is not available across the board (it should be), I would love it if I could earn open access credits for my publications in exchange for providing reviews.
In part it’s prestige, which for some might matter for promotion purposes, and at least personally I’m more like to cite journals for which I know I trust their judgement in peer review and submission acceptance. There are predatory publishers which abuse the open access concept to make money, and if I’m reviewing literature I don’t want to have to also research if a journal can be trusted (unless of course the publication I want to include is novel or especially worthwhile).
Also, in many contexts open access requires payment by the authors; this may be fine if an author is in a large grant-funded lab or at an institution willing to fund the open access fee but for many of us non-research-track folks it’s kind of a deal breaker.