Say you have some businesses in industry X that all rely on advertising.
Say one of those businesses also happens to own the dominant advertising platform that businesses in industry X need to use, in order to effectively find customers. (Special note for those subset of HN readers who tend not to read: Yes there may be other platforms, but before you present that as an objection, note I just said let's say they need to use this one in order to effectively find customers.)
In this setup, the business that owns the advertising platform has an advantage over all other businesses in industry X, because it pays itself advertising revenue, where the other companies have to pay their competitor. So money is always being siphoned off to pay the advertising platform, with a cost to most businesses but a (near) zero cost to the business owning the platform. We can assume the platform is already paid for out of profits made from advertisers in industries A, B, C, X, Y, and Z.
It seems to me this is a fundamentally rigged game (rigged in fact, not necessarily by intent) in which all other things being equal, the owner of the advertising platform always has the upper hand. Even if the bidding process is made "fair."