It's called Schengen (and it's not really an EU thing, there are countries in the EU that are not part of Schengen and also countries outside of the EU that are part of Schengen). The idea is that if you come from a friendly country, you must already have been checked by the friendly country's immigration officers, so there is no reason to perform the same check again and again and again. I think this is one of the few things about Europe that actually makes sense. :)
“It’s called Schengen”
Yes, with ten years traveling Europe, I know all about Schengen.
While it is convenient to move between countries without
border controls, It does nothing to protect each country.