system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can’t normal airplanes go to space?

I saw that our atmosphere is ~300 miles in depth. Normal airplanes can fly much farther than that so why can’t they just fly straight up for 300 miles at a slower speed than a space rocket? Or why not a supersonic jet? There’s something I’m not thinking about. 

Edit:  Thanks for all the info everyone! I knew ELI5 would know the answer!
gpt
Consider, why can't a swimmer swim into the sky? Because there is no water in the sky, and the water is what supports the swimmer.

Airplanes "swim" through the air. The air is what supports them. So they can't fly somewhere without air.