It's strategic from a military perspective, not economic perspective.

Very little of anything actually goes on in Greenland. It has a population of less than 57,000 and a GDP of less than $5B. The US maintains significant military presence, including airbases, missile launch and intercept capabilities, and ensures the US controls the North Atlantic, instead of Russia.

with apologies to https://xkcd.com/149/ : "national security make me a sandwich"