IMO, open borders are risky because not everyone trying to get through is a poor family or worker. There are a lot of bad actors out there, and you definitely don’t want people like that getting through without resistance. Think drug dealers, human traffickers, other types of organized criminals, including foreign agents. Sadly, the world isn’t just people operating on good will.
From what I’ve seen, the foreign border control is the main line of control, that’s why they separate lines into people with EU passports and other passports. It’s the same as moving without border control between US states 🤷♀️