Advertisements. It's no use advertising on other search engines if nearly no one uses that. The market is probably changing now that Facebook is now also interesting for ads, but there was definitely a period where internet advertising meant Google Ads.
Of course, asking to disclose Google's search algorithm borderlines insanity.
But there are certainly plenty of non-Google places and means to advertise. Google could raise prices on AdWords across the board and advertisers would have plenty of alternative means through which to hawk their wares.
Wouldn't you agree that the ad market via search is much different than the overall web ad market, especially when 90% of users (in some markets) go through search to find things?
No, I don't really think I would. Search is a great place to capture consumer intent, but it's hardly the only one. Advertising dollars are finite, and Google absolutely competes against non-search advertising publishers for its piece of that pie. Why do you think they run the AdSense program?
Even if it were a different market, Google doesn't hold a monopoly on it by a long shot. Bing is over 20% of search volume in the US, and Yahoo holds another 13%. Searching for products on either of those will return results for relevant advertisers; the simple fact that they have paid ads to run is an empirical contradiction to claims of a search advertising monopoly (or is empirical proof that a large number of very successful companies are economically irrational and are throwing money down a money hole, but that seems highly improbable).
Of course, asking to disclose Google's search algorithm borderlines insanity.