system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do planes have to go so high if there isn't anything going on 2 or 3 thousand feet up?

I don't understand why planes don't just fly around about 500 or 1,000 feet up if there is nothing there to obstruct them. Why do they have to go so high? Because of the noise?
gpt
Many reasons. Jet engines operate most efficiently at higher altitudes, which means you use less fuel, which makes flying cheaper. If there is an emergency it is best to have as much altitude as possible underneath you, you have longer to sort out the problem or decide what to do while you are gliding. Also, ever lived near an airport? Jets are LOUD. Flying at 35,000 ft is a lot more pleasant for the people on the ground. Traffic separation is another reason, different types of aircraft tend to be much more common at different altitudes. Most people don't realize just how many aircraft there are in the sky at any given moment. You have to space them out in order to avoid collisions and keep things moving smoothly.