Another reason why jet airliners fly at high altitudes - they were designed to.
At low altitudes, the greater deinsity of air creates more drag from just forcing the airplane through all those molecules. As you go higher, there’s less air, so less parasitic and form drag.
But as you go higher, the wings have to work harder, and this causes another form of drag - induced drag. This is the drag produced as a result of the creation of lift. When the curves for parasitic drag cross, you’ve found the lowest point of overall drag for that airplane. That’s your best cruising altitude.
One of the ways to reduce induced drag is to use a wing with a larger aspect ratio - long, thin wings vs short, fat wings.
Look at this overhead picture of a Boeing 747-400. Look at how long and thin the wings are. Now compare it to a picture of something meant to go low and slow, like my old Grumman AA1B.
You don’t want long, skinny wings on all airplanes because it makes them harder to manoever, harder to park and taxi, and heavier. So if you do your flying down low, you trade off high altitude efficiency for other advantages. But if you main interest is going a long way as fast as possible on as little fuel as possible, you want to design your airplane to fly as high as you can, subject to engines being available that work efficiently at that altitude.