Why don't search engines show more results on the first page?

The typical search engine user is said to not go beyond the 3rd page (beyond the top 30 results) of the search results pages. In fact, many don't even go beyond the first page.

Google and Yahoo both have the option to set the number of search results a user wants (haven't seen it on MSN -- may be it does exist). Why not set the default number of results per page to be 30 instead of the current 10?

That way, searchers will be presented with more organic search results, which should by definition be the most relevant results for a search user. The search engines could also display more sponsored results with a longer page.
Or do the search engines fear loss of revenue with more organic results on a page?