Why can't people do business , in their own country, why can't they give job , to their own people?

there are lot of factors relating to this question like: Economy, requirement, agreements with other countries. for eg: country A wants x from country B, but country B can not sell x to country A due to some reason, then Country C will enter in to agreement with country B and A to purchase and sale x. So to do this transaction country C may need to recruit people from Country D.