Because it's big and largely empty. Besides, I don't know about you, but when I think about stereotypical Argentine images I think of towering Andean peaks and wide open pampas and gauchos - not exactly imagery that makes you think 'crowded'. Patagonia is mostly just empty, wind-blasted treeless steppe.
It's an awful lot of desert. What is your point? Yeah, maybe there are parts of the country that aren't blasted hellscapes constantly covered in dust where taking a walk is a harrowing right of passage undertaken by the natives to prove how tough they are. But what about the rest of the country? There are reasons why most of these countries have low population density and the #1 reason is that they are mostly inhospitable places to live. Every single one of these countries have large amounts of land that are either frozen tundra, arid desert, dense jungle, or otherwise uninhabitable. Is there some reason you feel compelled to argue? You know what I mean.
I got them all without cheating. I started with the answer I knew was #1. Then I guessed Iceland because I know they have a very low population and I remember seeing them on such lists before. Then I moved on to guessing countries that were very large with relatively low populations... mostly the frozen, jungle, or desert countries I referenced in a comment above. That took care of all the rest in the Americas, Asia, northern Europe, and the Sahara. And finally I turned to guessing random countries in Africa since I know that African countries by and large, though growing rapidly now, still don't have the population density of most more prosperous countries- that got me the last few stragglers.