Why internal links are worth the effort
Internal links do two jobs at once. They are how a crawler finds pages — a page nothing links to is a page Google may never see — and they are how ranking value moves around your site, concentrating it on the pages you most want to rank.
They also tell Google what a page is about. The words in a link are read as a description of the destination, which is why "read more" links are wasted: they pass position without passing meaning.
The reason it does not get done is that it is tedious. Adding links means rereading old articles looking for phrases that match new pages, and nobody does that consistently. Automating the search while keeping the judgement is the useful middle.