The reason is (more or less) personal jurisdiction. If a company does substantial business in a state, people from that state can sue the company in that state under the laws of that state - no matter where they are based. This is a Good Thing - for example, say hypothetically that a Japanese automanufacturer makes all of their cars in Japan, and simply ships them to the US and sells them. If you a car from them and it explodes, injuring you, you shouldn't have to sue under in Japan under Japanese law.
See also: Article on conflict of laws for a primer on which jurisdictions laws apply when.