The only way to actually stop it is to get everyone to stop at once, via regulation. Otherwise, stopping by yourself is just a unilaterally bad move.
That's the charitable explanation, at least. These days I don't trust anything Musk says at face value, but I do think that AI is driving society off a cliff and we need to find a way to pump the breaks.
How will that work across national boundaries? If AI is as dangerous as some claim, the cat is already out of the bag. Regardless of any licensing stateside, there are plenty of countries who are going to want to have AI capability available to them - some very well-resourced for the task, like China.