Does it matter? Many scientists in the know say that peer review is as good as a trashy spam filter these days. This isn't to say that it is absolutely useless, more that it is very overrated.
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.