I used to wonder this too, until I started actually watching friends and family using Google.
I would imagine that most of us posting here, when looking for something using a search engine, will think in terms of what sorts of documents might hold that information and how they will have been indexed. So if we're looking for a quotation we might put those few words we know are definitely correct inside quote characters, perhaps with the word 'quotation'. If we're looking for something that might be referenced in a forum discussion, we'll think about how other forum posters may have referred to it and chose keywords they're most likely to have used, ignoring words that would produce too many hits or ambiguous results. If we're really trying to narrow things down we might even use the site: function or other advanced Google tricks.
On the other hand most non-technical people I know don't really understand what Google is or how it works. They just see it as a magic information portal that answers questions. So they type something completely inappropriate or generic into the search box, full of words like 'how', or 'why', or 'where' but with no specific context to help the algorithms narrow it down, and are disappointed when a load of random links is returned. Worse, they will often then click on the first thing that pops up -- more often than not a sponsored link to something completely irrelevant -- then complain that Google "isn't working".
There was a time when I would have said the answer is better user education, but then if folk can't grasp the real fundamentals I can't really see what sort of education would work for them. For heaven's sake, users are still having to be reminded by printed publications that URLs are typed into the address bar and not the search box. Even the advent of 'awesome bars' to try to mitigate this led to disaster, as witnessed by the ReadWriteWeb / Facebook login fiasco.
Perhaps there will always be a divide between people who understand the nature of a particular tool and those who never will. A hammer is one of the simplest tools ever invented, so simple that anyone should be able to use one. Yet while carpenters and skilled DIYers use them to create things of function and beauty, all some people can manage to do is bang holes in the drywall and bruise their thumbs.