Those two skyscrapers would have been making you a fortune for decades, would be making you a fortune each year now, and would continue to do so, and when you sold them to some Russian oligarchs, would make you an even more enormous fortune than all of that put together.
In contrast, 'the biggest island in the world' is a worthless superlative as about as meaningful as "biggest ball of twine in the world", and it is useless and expensive to maintain and has not, is not now, and probably never will make you a fortune. Can you eat 'biggest island in the world'? Does mere bigness make it useful? Is the Sahara valuable solely because it is large? Do people prefer to go to Greenland rather than Hawaii because it's so much larger an island?
Even if Greenland does start seeing real economic activity - maybe that much ballyhooed oil will finally start being economically feasible - and it makes you specifically a fortune rather than someone else, it'll do so in some distant unforeseeable future and you will have lost out on everything you could have done with that money now. If you think that's a good deal, I have an offer for you: send me $100m now, and I swear I will return you $100m in 50 years. Good deal, right? Who knows what you can do with $100m 50 years from now!
> And Denmark is not exactly a country with vast tracts of unused land.
"Denmark has a population density of 131 people per square kilometer (339/square mile), which ranks 88th in the world."