Because search is hard. Most domain sites search titles and descriptions of articles for keywords. Google will index the entire page and be able to provide more and better ordered results.
Or use to. Now the quality and quantity of results forces site owners to implement their own search or use third parties.
This is still the case for plenty of websites. Searching google with "site:example.com search terms" often still gives better results than using a site's own search tool.
Or use to. Now the quality and quantity of results forces site owners to implement their own search or use third parties.