When you use Google you are generally actively looking for something. When you use Facebook you get ads for things you aren't looking for.
When you use Facebook you get ads for things you are looking for in a broad sense, they just happen to figure out what you are looking for via fingerprinting and spying on all your web and app activity (to the maximum extent allowed by platforms and regulators).
Google, of course, also does this in addition to looking at your currently active web search.
Google looks at both your current search/browse activity and gathered data (the gathered data being especially important for all the very many Google-served Ads off of google.com)
Facebook also looks at your current search/browse activity and gathered data.
The only real difference is Facebook search is universally entirely worthless so far less of that signal gets used relative to more passive snooping.
Either you have intent but little attention (Google et al.) and so can charge a decent amount.
Or you have lots of attention but little intent (Facebook et al.) and so can charge a decent amount.
You can't charge a decent amount if you have little intent and little attention.
OpenAI presumably has intent, but hasn't done all the Google-esque adtech sausage making to expose that to ad space buyers.
Let's say a search term like "apartment for sale in Dallas", I think 15 years ago one of the highest grossing search keywords was something like "asbestos cancer lawyer".
And Meta has incredible amounts of behavioral data but is usually not that close to the thing I want to do right in this moment, they closed that gap.
Open AI has a very good psychological profile, very little current intent and very little attention, but this might change.