People shouldn't be trapped by accident of birth—if someone can improve their life and contribute elsewhere, they should have that right. Migration creates economic growth, fills labor shortages, and brings cultural diversity that benefits host countries. Restrictive borders are ultimately an arbitrary way to protect privilege and keep poor nations poor by draining their talent.
Completely open borders would overwhelm public services, tank wages for native workers, and erase the distinct communities that give people identity and belonging. Countries need to control who enters to protect their social contracts, maintain cultural cohesion, and ensure resources go to citizens first. Managed borders are how nations stay stable and functional.