Why do people treat nationality as an accomplishment?

The point can be made that if you vote in a countries elections. Your shaping the future of the country.

The point can be made, that if you work in a profession of any kind, your contributing to society of the country. Stimulating the economy by paying taxes etc.

The point can be made, by having children who will grow up to be future taxpayers, that your contributing to society because those taxes help the country

The point can be made by giving your child education. That they grow up into a professsion which hels the country.

All these are accomplishements, and people should be proud. Because its for said country.

Your looking at it in the wrong perspective.

Same reason half a billion people watch the superbowl. Everybody loves the superbowl for no other reason than everybody loving the superbowl. Not liking ubiquitously favored institutions will get you pushed to the fringes of society about as quickly as running around naked out side your house and cursing at traffic.

Because they are lacking an identity in their life, so mask their insecurity hiding behind a greater generalisation mass, like a country

They're proud to be a small part of a unit that is much bigger than themselves. But, meh. What can you do about it?