Because greed.
The examples for the dangers of AI is always comparisons with tools designed to destroy (which is understandable). But the problem with AI is it has the potential to create wealth for investors.
Thus those who have the opportunity to change course also have a conflict of interest in making that decision.
There's also a zero sum, race dynamic between geopolitical rivals. The US will not permit China to win anything if they can prevent it. The destruction of humanity is only a possibility and China will continue to develop it anyway if the US slows down.
So the choice is: negotiate an unverifiable treaty (I.e. there's no way to verify compliance), or keep going as you are and try your best to not cause the destruction of humanity without slowing down.
Greed can be counter balanced by liability. It can also be limited by removing market access. For much of American history you couldn’t get a corporate charter for multiple states, you were only allowed to operate within one state. Imagine if Sam Altman was only allowed to serve inference to customers in California and legally prohibited from setting up other companies and running inference in New York, someone else would then set up a competing business in New York. Market access is a choice and people are so used to liberalized commerce that they forget the restrictions that are available and existed in the past.