I like to think of "peer review" in the same way as "peer code review". Just because someone looked at it and approved it doesn't mean that it is free of shortcomings and defects.
That's a pretty light annual workload. How do you get to be one of those people?
Or do those people have other work to do?
Yes, they have other work to do.
I'm pointing out that the number of papers someone reviews in a year is not even distantly related to the amount of attention they have available to devote to the job.