It's kind of a clever solution by Google, if a bit troll. They can make that ad auction 100% fair and still win it because they have more expertise on running efficient ad campaigns than anyone. IMHO that just doubles down on the danger of their monopoly power, but perhaps it complies with the antitrust requirements.
They know for each user every other search result clicked. They know what the users interests are. They know which users prefer stuff delivered fast, which prefer the cheapest items, and which will be wooed by 50% off deals.
They can use that data to present the best possible results, get the highest click through rate, and sell the most products.
Therefore they can afford to pay more for the ad spots than someone without that data, who gets less precisely targeted ads, and fewer product sales per advertisement shown.