[WinstonSmith84]

This is so naive... let be honest, it's just pure nationalism, the pride of each country thinking to be better than the neighbours.

Let's take some examples of some "flagship" western European countries, such as Germany or the UK. These 2 countries have constantly classified the risk level of other countries, forbidding/warning their citizen to not travel abroad in "risk areas", despite themselves having, at times, much higher infection rates than any other country they classified at risk. It was specifically the case in the first half of 2021.

A counter example, the US is as big as the whole of Europe and they didn't "compartimentize" their states like it happened in Europe. And they just did ok (compared to Europe)

One has to realize that Europe is a patchwork of countries which have always fought against each other since history remembers, and "Schengen rules" is a new thing which is not deeply changing the culture and instincts of these countries and their citizen.