Wouldn't it be cheaper and better for the environment to buy some land off Germany or Sweden? Once the deal is sealed, they could start building the city the very next day.
Yeah, countries are usually unwilling or incapable of turning over their land to another country.
Back in 2016, the Norwegians had an idea to gift Finland a new highest peak by shifting the border between Finland and Norway just a bit as Finland was due to celebrate its 100th year of independence. However, it never actually went anywhere since it'd be unconstitutional for Norway to start splitting off its country.
This is to protect the local port for the capital of the country, particularly during large storms. Buying land inland along the border would not accomplish this protection task.
There is plenty of space in Denmark for its existing population (and for millions more, for that sake). The issue is that there is not that much space in Copenhagen.
There is plenty of space in Copenhagen. However plans to build anything anywhere gets shot down constantly with silly laws and restrictions. Politicians don’t want a more healthy housing market in the capital because they are all some of those who are profiting most from how unhealthy it is.
Because they want to sell expensive housing close to the sea, not affordable housing far from the sea.
It's a stupid prestige project, primarily designed to attract rich investors, who will let their investment object apartments sit empty, creating another dead neighborhood.
It is definitely an idiotic and corrupt project, but how would it help Copenhagen to buy a part of Sweden? There's plenty of farmland in Denmark that could be used, including within bicycle distance of the center.