Google usage for search seems to be quite a bit North of 90% in the most populous parts of Europe, and slightly above 70% in the US. There probably is a reason for that, but it is not that other substantially similar services are unavailable or more costly. My hypothesis, based on fairly unsophisticated sampling and analysis every month or two, is that Google's results are just a bit better than those of Bing,Yahoo, and DuckDuckGo. They are not a lot better, and I gave DuckDuckGo a more extended trial last year as default search engine but switched back to Google after about three weeks; the results were not quite as good as I had hoped.
So Google has a monopoly of search, certainly in Europe and arguably in the US, in a market in which they were not the first competitor and in which consumer choice is as completely free as possible, given that none of the commonly used search portals charges users.
Google makes money in large part by selling advertising and displaying the ads to its users, as do others. Its customers are those who pay it for advertising placement, not those who use it for search, even when they are looking for something to buy. They charge their advertisers a fee, and presumably have fairly specific contractual obligations to their customers, the advertisers. They do not, as far as I know, have a contractual or other obligation to those who use their search facilities, but they do have a moderately strong self interest in providing search results that meet the perceived needs or requirements of those users, since failing to do so will cost them search share and consequently reduce the the rates they can charge advertisers, and their profits.
Why, exactly, should they not place the links for products and services they offer immediately beneath those of their paying customers, and above those for every single other vendor of comparable products or services?