New users to the Stack Exchange communities likely just don't know about the reviewing option, or meta for that matter.
Also, despite SE's stated "be nice" policy the simple fact is that new users are often rudely responded to and dismissed by established users and moderators with tens of thousands of reputation points. This discourages community participation as much as it separates the wheat from the chaff. The new users that stick around long enough to do more than get their question closed or actually obtain a useful answer seem to take on this habit on squashing any interest which new users might otherwise have in participating in, maintaining and reviewing the community.
Looking at the total list of users (only 4.5 pages out of 141 list users with more than 350 rep points), questions, and questions where badges are earned for nice or good questions, especially if pets.SE is the only site the new user is familiar with, it simply takes a while to get 350 rep points (with or without the 100 points for being a trusted SE member elsewhere).
I'd say it's as much a function of any beta site as it is the SE model. Those that do get 350+ and stick around long enough tend to become reviewing members eventually, but it takes a while to develop the core community in a beta site. Of note, tho: all of 369 Custodian badges have been awarded. In a community with 5,851 members... that does seem low. Perhaps expanding the Custodian badges might help in beta communities, but that is a question for SE.meta.