I've seen some journals do this. The problem TBH is that the amounts paid aren't enough to be an incentive. Typically £50-100. A good review takes a day to do (maybe more). And academics are probably paid £100/day or more anyway, and aren't very money-motivated.

I think a better approach would be publicity. Academics care about reputation a lot. If good reviewers were acknowledged by the journal, that would be a big incentive, something you could put on your CV.