There is no physical wall at 42,000 feet. The ceiling is made of economics and air. Jet engines need oxygen, and the higher you climb the thinner it gets; wings need air to push against, and the gap between the speed that keeps you flying and the speed that tears you apart narrows to nothing — pilots call it coffin corner. Airliners are certified to a service ceiling near 41–43,000 feet and almost never use it; the efficient, legal, comfortable altitude is the shelf you see bulging in the cross-section. Above it fly only business jets and the odd outlier.
Below the shelf the sky looks busy on the map, but most of those low dots are clustered around airports — planes in the first minutes of a climb or the last of a descent. The long middle altitudes are nearly vacant: nobody cruises at 18,000 feet. So the air traffic of the entire planet resolves into two thin layers — a scatter near the ground and a dense ceiling-hugging shelf — with a hollow in between. At a rough ~90 people per flight, the planes on screen are carrying on the order of half a million human beings, nearly all of them sitting within a few thousand feet of the same height.