Q: Why is a PhD essential to become a peer-reviewer

I am going through the process of becoming a peer-reviewer, while I am coming across every journal editor mention that being a doctorate is a minimum qualification, to be approached for reviewing. I have many peers who have had a good amount of author experience who are eager to don the peer-reviewer hat and contribute on the other side of academia as well (including myself). In this context, I would really appreciate if editors/associate editors here provide insights on why a PhD is essential to become a part of the peer-reviewing process.