It depends on what you mean by "open."

Yes, the reviewers' comments and the authors' rebuttals are generally confidential, and there are good reasons for doing it that way. One significant reason is to allow the reviewers to make their comments freely, without worrying about how those comments might be perceived by the public or how the authors' reputations might be impacted; likewise with the authors' rebuttals. Another reason is to avoid tainting the review process in situations where a paper is rejected from one journal and re-submitted to another -- the second set of reviewers should not know that it was previously rejected, nor should they know why it was previously rejected.

On the other hand, the peer review process is "open" in the sense that there is no explicit credential required to be a reviewer, only expertise in the field (typically determined by publishing papers -- in principle someone with no formal education could publish enough papers to start getting invitations to be a reviewer). It is also "open" in the sense that anyone can submit a paper for review as long as it adheres to formatting (page layout, font size, etc.) and length requirements.