Reviewers don't always have time to do good job. Some are incompetent. Others have an agenda, such as saving the world from climate change, or preventing anti-vaxxers from having more motivation not to vaccinate. However well motivated you may think these agendas are, they conflict with Scientific progress. Others may have agendas I Like not publishing papers that are conflicting with papers they are about to publish, or favoring certain institutions, for altruism, or racism.
The link below presents a computer model showing that even a small fraction of otherwise motivated reviewers leads to a substantial degradation in the quality of published papers.