I suspect in the near future (if not already) ChatGPT data will be sold to data brokers and bough by Amazon such that writing a prompt will end up polluting Alexa product recommendations within a few minutes to hours.
So, this works only for a subset of ads, specifically, ads that are CPC (cost-per-click) (sometimes called CPL -> cost-per-link), this wouldn't work for CPA campaigns (cost-per-action), which only pay out when a sign up or some other arbitrary action is performed by the user.
The larger adtech companies will have some form of bot-activity-detection, _but_ plenty will let it all just slip on by. (I've personally written a Babashka script at $job that checks performance of our publishers, and flags any fraudulent-seeming activity)
Watch Adam Curtis Century of the Self. I don't think he set out to explain ads in tech, but to my mind it answers a lot about how ads specifically came to power.
when there's a need to make money, but there's noone who wants to pay (but they want to use the service), then it naturally falls onto being ad-driven.