system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do commercial airplanes have to fly at around 35,000ft? Why can't they just fly at 1,000ft or so and save time on going up so high?
gpt
Fuel economy is much better at high altitude, with lower density air.  Additionally, altitude allows for recreational flying, local flying (e.g. helicopters etc., balloons, etc.) to use lower altitudes safely assuming safe distance from airports).  It also keeps the visual pollution of the amount of airplanes in the sky such that we basically don't experience airplanes above us even though there are a TON of them.  At 1000 feet, people on flight paths would really notice!