There are only so many ad dollars to go around. ChatGPT getting traffic doesn't mean they can convince ad buyers that their dollar goes further than at established players like Google and Meta. AI search is a commodity at this point, even DuckDuckGo has it.
Reddit has been one of the world's top websites for over a decade, yet they are totally irrelevant in terms of ad product market share.
Because reddit users are low quality users. Meta and Google can peak into your data and segment you to sell.
ChatGPT ads are low quality placed at the bottom barely noticed.
ChatGPT ads will get better. Meta's social userbase is drying up. Google keeps introducing and removing things from chrome to keep access to spy on you for them alone. We'll see how things pan out but in ten more years reddit ads will still be worthless.