sovereign countries can NOT declare bankruptcy and refuse to pay
What are you talking about? That is the definition of sovereign. Of course they can refuse to pay. They just can't expect favourable interest rates afterwards.
Argentina is kind of a bad example. They had huge inflation through the '80s, then locked their currency to the US dollar through the '90s to compensate. This made their position much worse, as they needed to be able to devalue their currency to keep what industry they had competitive. The default was just an unavoidable consequence of decades of economic stagnation and bad policy. They were screwed before the default, and surprise, surprise, are still kind of screwed after.
It's just a trade. You trade your ability to borrow money in order to drop the obligation to repay past debts.