Except that there are hundreds, maybe thousands, of niche advertising platforms, markets, middlemen, services, systems, technologies, clearinghouses, inventorie

Except that there are hundreds, maybe thousands, of niche advertising platforms, markets, middlemen, services, systems, technologies, clearinghouses, inventories, sellers, buyers, brokers, verifiers, subverters ... The advertising ecosystem is actually very complex and diverse. Nobody is quite sure how big it actually is. But Google definitely has the two biggest chunks of it and nobody else is able to, for example, place keyword-based ads into Google's search results without Google taking their cut.