"The best" is not a static product. If Amazon had no way for vendors to showcase their product, the best would be a stale product that accumulated decades of reviews. Maybe it's still the best , maybe not. But there is value to show some noise in the search long term.
Another way to look at it is through the vail of ignorance. Suppose you're a manufacturer of air fryers, but don't know where you stand in terms of how competitive or popular your product is. Would you want to live in a world in which you CAN'T bump your placement? Sure if you're the top 3 manufacturer, maybe you would prefer not to pay the tax. But if you're not, you'd much rather prefer the system in which you can.