system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why were ridiculously fast planes like the SR-71 built, and why hasn't it speed record been broken for 50 years?
gpt
Before we could use satellites to take pictures from space, if we wanted to see what was going on in enemy territory, we had to take pictures from a plane.


  
Enemies didn't want us taking pictures so they would try to stop the plane - usually by blowing it up with missiles.

  

We didn't have "stealth" technology yet to keep from being seen, so if we wanted to avoid getting hit with missiles, we needed a way to avoid them. The best way we could come up with was to go so fast they couldn't catch up.

  

Being really high in the air helped this, because it's easier to go fast up high, and because it would take missiles so long to get up to you, you could be out of the area before they reached that height.

  
As a result, the SR-71 was designed to go as high and as fast as possible.


  
Since then, we learned to build space satellites to take our pictures, which can't be hit with missiles. We also developed "stealth" technology for planes, which keeps them from being seen on radar. This means we no longer need to develop planes for high and fast work, so the SR-71 remains the best at that.

(Edited to remove error related to a missile strike)