"Why do we need to demand social justice for workers in India or China, if we never met one?"
Because that social injustice makes them unfairly compete with our offspring for jobs and has reduced potential growth of our own paychecks for 2 decades already. The loss of mass-manufacturing in Europe and North America is a major contributor to unpleasant political developments. Therefore, it is in our own interest to support social justice in Asia. Another reason is to prevent undesirably large streams of migrants.
As lab manager at manufacturing service factories in Thailand since 2004 and in other jobs already since 1996, I have unintentionally been supporting economic globalization. I am pleased to see how much social progress local engineers have made but I am very dissatisfied with the meager progress of workers' rights and prosperity at lower levels. That lack of progress has certainly contributed to the dramatic drop in Thailand's political stability since 2005.