You are confusing the number of authors with the number of peer reviewers. The authors are placing their names and reputations on the paper and its quality/results. You have no idea how much input the site PI had to the paper. They may have reviewed the drafts and agreed with the paper, its conclusions and decided that was enough to place their reputations on the paper or they collectively may have had input to how it was written and the conclusions made.
The peer reviewers don't put their names on the paper. There are typically 2 to 3 reviewers, They are gatekeepers for the journal charged with quality control and preserving the reputation of the journal.
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