John Mueller of Google wrote a very detailed and honest explanation on why Google (and third party SEO tools) do not crawl and index every URL or link on the web. He explained that crawling is not objective, it is expensive, it can be inefficient, the web changes a lot, there is spam and junk and all of that has to be taken into account....
-
Google Explains Why Google Does Not Crawl & Index Every URL
seroundtable.com