Answer:
The US is the largest economy in the world, but it only represents about 1/4 of the total. That means that the opportunities of earning higher profits and effectively using all their resources increases dramatically when you serve the whole world. E.g. Apple is the mot valuable firm in the world and about 60}% of its revenue comes from foreign markets. Something similar applies to most large corporations, that would be much smaller and less profitable is they only served the US market.