They have a service which understands a users question/needs 100x better than a traditional Google search does.
Once they tap into that for PPC/paid ads, their profit/query should jump into the green. In fact, there's a decent chance a lot of these models will go 100% free once that PPC pipeline is implemented and shown to be profitable.
If they start showing ads based on your prompts, and your history of "chats", it will erode the already shaky trust that users have in the bots. "Hallucinations" are one thing, but now you'll be asking yourself all the time: is that the best answer the llm can give me, or has it been trained to respond in ways favourable to its advertisers?
Google used to segregate ads very clearly in the beginning. Now they look almost the same as results. I've switched to DDG since then, but have the majority of users? Nope. Even if they're not using ad blockers, most people seem to not mind the ads.
With LLMs, the ads will be even more harder to tell apart from non-ads.