Because Google is quite a bit better in the “type random words into search, some of them misspelled or missing entirely, and get a decent search result” department.
That’s why.
The budget for that sort of effort is many orders of magnitude over what we at Discourse can bring to bear on the problem. Google has 57,100 employees many of whom work on search. We have.. eleven.
If you end up on 404, you are sending a strong signal that you need serious help. So we offer serious help.
(We do suppress this part of the page for private sites, obviously, as google can’t see into private sites. @techapj added that feature a while back.)