Chrome/Chromium's value is in the development momentum. You can pile up features, manipulate the entire web, and make it impossible to compete for others. This is what they're trying to buy. A fork isn't enough - there's a huge difference between a technical fork and a meaningful fork.
That's a strength of Chrome for Google. OpenAI would not enjoy the same benefits as they don't control the whole vertical. They want the 4 billion strong spyware botnet that is Chrome.