I'm going to change up the countries so there's less emotional value associated. Why should Polish people leave their country so they can find work in England?

I'm going to change up the countries so there's less emotional value associated. Why should Polish people leave their country so they can find work in England? In theory, countries should be self-sufficient enough to where only niche based jobs should have such movement. I'm talking about highly specialized stuff. But when it comes to generic labor, why? Farming? Building houses? The fact people want to leave one country for another just for basic jobs is kind of messed up. I feel both countries are to blame forcing people to leave their homeland. One needs to get their fucking shit together and/or quit punishing a demographic because said gov wants to become a more service oriented country, the other country needs to stop creating incentives for such actions to become easily profitable on a large scale basis. Countries need to be self-sufficient with their own population. One population sacrificing its people for the good of another is not sustainable, no matter how you slice or dice it.