Google’s John Mueller said that it is impossible for Google to index the whole web. John Mueller passes this statement when it was asked from him that why Google does not index every page in a sitemap for a particular website.

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Reason:

The main issue is that new pages arise all the time. It is found that sometimes these pages are for relevant purposes and sometimes not. In fact, Google does not want to index all the pages on the website as they are spammy, low quality, and subpages. Google doesn’t want to waste its time over crawling, indexing, rendering these pages.

Tweet Made By John Mueller:

Which issue do you mean, "indexed, not in sitemap" or like Martin mentioned, the opposite? We really don't index all pages in sitemaps or on the web, it would be impossible to do so.

— ? John ? (@JohnMu) November 27, 2019