National boundaries weren’t as strict for the working class until around WWII, though it obviously varies by region. It’s more attached to nationalism, the ability of nation states to influence “their people” who moved to other countries to engage in sabotage campaigns and the paranoia over this kind of activity that followed this.

One of the requirements of a free market capitalist system is free mobility of labour. So it’s not really a class thing, it’s a nationalistic thing.

I mean if I’m an evil businessman, why would I want to limit the number of people competing to get a job?