They have to build out all the backend ad tech crap that google and fb have built over decades. As in the campaign tools, scheduling, targeting, analytics, auctions, exchanges etc. Advertisers and publishers have already deeply integtated into that eco system. Also the google and fb system has created a content generating army that is incentivized to fight tooth and nail for eyeballs. OpenAI doesnt reduce the activities of the eyeball capture army, it just freaks them out and pushes them into overdrive.