Nvidia is obviously a US company, but the chips themselves are manufactured in Taiwan.
And of course China sees Taiwan as a rogue province, at at least treats invasion there as "on the table". While the US may decide to actively support Taiwan, at the very least a war over that will disrupt production.
Should Taiwan fall, and China choose to ban exports from there to the US, then the fun and games would really start.
As long as back-channels exist to supply the chips to China, then there's less incentive for China to control Taiwan. The ban exists as good politics (we don't ship to our adversaries) while the back-channels ensure they aren't forced into a position no-one wants them to be in.