It depends on where you are flying from, but the answer is because the distance is most likely shorter.
The Pacific Ocean
For safety reasons; it's so that they don't have to fly over eastern Ukraine following the shootdown of Malaysia Airlines Flight 17. Many hundreds of flights that used to fly over Ukraine have been diverted away for this reason.
No, you would fly over the states. 20 hour flight is still much shorter than flying over Europe and Asia (ie, the length of russia)
They don't sometimes.
The Atlantic Ocean.
Scandinavian Airlines fly to many different places. They don't fly everywhere in the world, but they do fly to Europe mainly. You can fly out from there and then catch a connecting flight.
hot-air balloons
If you flew due south from Chicago you would not fly over a South American country. Instead, the plane would fly over the Pacific Ocean.
Napoli is Naples, in Italy. Flying southeast from Naples you could fly over Mount Vesuvius or Pompeii and eventually out over the Mediterranean Sea towards the north of Africa.
Yes, you can fly domestically with a passport instead of a Real ID.
Amelia flew to many places in the Americas and Europe. She was flying over the Pacific Ocean when she disappeared.
Look on a globe and you will see that Greenland is on the shortest route between many parts of Europe and the North American continent. The Atlantic is not the most 'direct' route.