My incomplete understanding is that publishing in big-name journals provides prestige and improves funding prospects for academicians. Since academia is very competitive, researchers will do whatever it takes to publish in the most presitgious journals. In other words, they provide a brand that researchers want to associate with; analogous to how rappers mention luxury brands (sometimes with but often without being paid) in their songs.

I don't know what benefits reviewers receive, but they are gatekeepers to the journal's brand, so conceivably they are able to obtain some benefit to themselves.