Because it changes all the time. A few weeks ago, it was Gemini 2.5 Pro, then Claude Opus 4.1, GPT-5 Thinking, now maybe Claude Sonnet 4.5, etc[1]. Having a good model isn't enough when they're basically interchangeable now. You need something else.

[1] This is an example. Which model was the best when is not important.

Because it depends on how much better “best” is. If it’s only incrementally better than open source models that have other advantages, why would you bother?

OpenAI’s moat will only come from the products they built on top. Theoretically their products will be better because they’ll be more vertically integrated with the underlying models. It’s not unlike Apple’s playbook with regard to hardwares and software integration.