We live in a society that has systems of market competition between companies and economic/military competition between countries. There's not just one country working on frontier AI, and not just a few companies in each country either. The incentives of these systems ensure AI advancement will happen quickly from here on out, no matter what, absent governmental action.
An individual researcher leaving the company does nothing, other than give their seat to someone else who likely doesn't share their same concerns.
Alternatively, using the influence they do have in their position to directly try to advance AI safety research, advocate for govt action, and make public arguments attempting to persuade the public to pressure the govt to act when they otherwise wouldn't be inclined is far more likely to impact things for the better.