The OPR is a KPI calculated similarly to Googles PageRank. Therefore, pages which get many links tend to have higher OPR than pages with only a few links. As pages with a high amount of incoming links, the probability is higher that a user will open this page. The user will in most cases not recognize he has been redirected. But a crawler counts every redirect and canonical as a step that could have been avoided.
This is why pages that get many links should be indexable.
In the report of non-indexable pages, there are two common options to fix those sites.
1: You make those pages with high OPR indexable, so either remove the redirect or canonical to another page, BUT you need to be aware of the danger of duplicate content. So this option only is to handle with care.
2: You need to check from where on your domain the non-indexable site is getting links from and change them to the redirect or canonical target page. This would mean that you are reducing the OPR of those pages to increase the OPR of the pages you wish to get indexed.