Off the top of my head, I can only think that the less dense air causes less drag. Is that it?
Do the engines lose any efficiency from having less air pressure to work with?
There’s also less turbulence.
Jet driver here…
You hit it exactly. Imagine jogging through ankle-deep water at the beach. Now try it in knee-deep or waist-deep water. BIG difference. Flying fast at low altitude is like jogging in waist-deep water. At airliner cruise altitudes we’re fighting against only 1/3rd to 1/4th the air density, causing 1/3rd to 1/4th the drag for any given speed.
The engines don’t lose efficiency, but they do lose power. In fact what limits altitude is the loss in available power eventually becomes so big that the engines are just unable to push the airplane higher. That’s not true in all aircraft; sometinmes other factors become limiting before engine power, but in general that’s the story.
The long climb to altitude does consume fuel, but we also gain a long coast downhill at the other end, so the net cost of the climb+descent is not much more than the cost of the same distance cruise at mid altitudes. The fuel savings at high altitude cruise more than offset the climb+descent cost getting there.
I think it is all in the airplane design. That is, you design a given plane (engines, airframe) to work most efficiently at a given altitude and speed. Some military jets are designed for low level work and probably do not fly as efficiently at higher altitudes. So, if you have a passenger jet flying at 35,000 feet most of the time you build it to work best there.
Don’t passenger cruise in a relatively large range of altitudes though? I know I’ve been on some flights that were significantly higher then others.
I am not a pilot but every passenger jet (not a puddle jumper) I have been on seems to fly anywhere from 31,000 - 40,000 feet (IIRC a 747 I was on cruised at around 42,000 feet but it was a long time ago and a hazy memory). I am guessing the choice of altitude has to do with traffic control and more likely weather (to avoid turbulence or make use of favorable air currents, etc.).
Another significant point is that jet engines gain efficiency as the temperature drops, which it does up to the tropopause. The height of this varies with latitude, but it’s typically around 9 to 14 kilometers - about where airliners cruise.
There are also some beneficial and fairly regular and predictable high-speed air currents at high altitude. Secondly, flying above the clouds an aeroplane isn’t susceptible to the chances of rain snow and sleet. But these are incidental. It is almost entirely the lack of drag, as others have said.