Because being a peer reviewer shouldn’t be done for free? Do you like being paid to work? Because Nature has established themselves as a premier journal over the the past 150 years and are known for their quality?
Peer review is not paid work. Mostly it is done as an overhead that is understood as part of being an active researcher.
I would even venture to say that most researchers will be against the idea of paid reviews, just like Amazon found out that paid product reviews are a very bad idea.
Or maybe I’m just taking crazy pills..