To show ads you need people to stay on your platform. This is especially true once ads become more intrusive or of lower quality, something the big players seem to gravitate towards to keep revenue up. Google and Meta have ways to lock in users (networking effects, the best search engine available, having your data stored there).

I am not sure if OpenAI has that. Their edge regarding models is small, their strategy currently seems to be "buy ALL the hardware so nobody else can". Users can quite easily switch to other models.