system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do airplanes collide midair if there aren’t that many planes, and the sky is huge?
gpt
Many fights come from and go to the same airports, which means that lots of planes are using similar flight paths.  If they were freeform flying from anywhere to anywhere, then there would be much less risk since they'd be following a million different paths.  But since there are relatively few starting points and ending points, more traffic is compressed into less routes.