Because migration restrictions prohibit them from hiring those same low-wage workers domestically. Is it really migration restrictions? Or is it arbitrage on

Because migration restrictions prohibit them from hiring those same low-wage workers domestically. Is it really migration restrictions? Or is it arbitrage on minimum wage (which may not even exist somewhere else), environmental regulations, worker safety rules, etc.? If developed western nations threw open their borders, they wouldn't simultaneously repeal all that stuff.