The entire stock market is premised on the stability of the US government. Without it all their wealth would disappear overnight. All the luxuries they love would cease being produced. They wouldn't be able to fly their private jets anywhere.
In the past the rich could stockpile easily-tradable goods like gold in order to maintain a luxurious life even if their government collapsed. When it comes to billionaires that's not possible. The logistics of keeping and moving that much physical currency/gold/etc don't work out in their favor.
If they keep this up they're going to lose almost all of their wealth as the world destabilizes. They're also setting themselves and their families up to be assassination targets for the rest of their lives (far, far beyond what they are already). There's people everywhere that will be severely impacted by their actions. There will be nowhere for them to go because the US really is the pillar of the world's economies.
