Trust in LLMs is easily broken, and many users are starting to see the cracks. Once those AI companies start rolling out ads inserted in the answers, the quality will go down even more, and they will burn the last good will of the people.

There is no moat because their only way to make money is to self-destruct.

Talking on a more practical POV, your cost to display the ads needs to be lower than what companies pay you for advertising. And while companies might be willing to pay a small premium for "better" targeting because the LLM supposedly has more personal data about users, the cost to deliver those ads (generating answers via LLMs) is several orders of magnitude higher than for traditional ads served on websites.

So even sticking to a purely technical aspect, ads might simply not be profitable when integrated in LLM answers.

Combine the two aspects, and OpenAI is all but a dead company.