So they'd file suit in the EU and... what would happen? Some sort of judgment that would prevent a company from starting business in the EU at a later date?
And similar arguments could be made for why American companies would play nice to the EU.
If your company was unable to comply with GDPR, many wouldn't want business with your company anyway, especially other companies who are trying to comply with it.
Except for military force and incarcerations per capita, the US is lagging behind in almost every statistic compared to China or the EU.