system
Given the following post, respond as if you're commenting on Reddit:
human
Why do airplanes need to fly so high?

I get clearing more than 100 meters, for noise reduction and buildings. But why set cruising altitude at 33,000 feet and not just 1000 feet?

Edit oh fuck this post gained a lot of traction, thanks for all the replies this is now my highest upvoted post. Thanks guys and happy holidays 😊😊
gpt
There are generally a few reasons.  One of the biggest being that higher altitude means thinner atmosphere and less resistance on the plane. 

There's also the fact that terrain is marked by sea level and some terrains may be much higher above sea level than the takeoff strip and they need to be able to clear those with a lot of room left over.

Lastly, another good reason is simply because they need to be above things like insects and most types of birds.

Because of the lower resistance, at higher altitudes, the plane can almost come down to an idle and stay elevated and moving so it also helps a lot with efficiency. 

Edit:  Forgot to mention that weather plays its part as well since planes don't have to worry about getting caught up in the lower atmosphere where things like rain clouds and such form.