Indexable pages with less than three incoming links are reported because we are considering indexable pages as important. If you want pages to be indexable you also want them to rank and you also want users to find the requested information on those pages. Therefore these pages should be strongly internally linked by other pages. Strongly linked pages have a higher OPR and also are more likely to be found by users.
If you want to check out the incoming links of a site you can just click the magnifying glass which will lead you to the URL inspector.