Because the prospective employees want to live in the US and the current legal climate allows that. Since you can't up and move to the US (from most countries) without a sponsor, the employment is the arrangement that allows this.
Because people are not automatons, there are often situations where a company has offices in, say, New York and Zurich, and some employees in the New York office moved from Zurich and some employees in the Zurich office moved from New York.
Because people are not automatons, there are often situations where a company has offices in, say, New York and Zurich, and some employees in the New York office moved from Zurich and some employees in the Zurich office moved from New York.