The legendary Rocket Launcher is a high power explosive firing weapon. It is a weapon created for the sole purpose of BLOWING STUFF TO BITS! In other words, everyone wants one of these.
The Rocket Launcher was invented by Master Chief in 2553. He made it when he needed a weapon to blow things up even better than he can with the Spartan Laser. He came up with a rocket-firing device. It turns out, the rocket launcher was not as epic as the spartan laser, so he threw out of the spaceship. But it went through a wormhole.
The rocket launcher traveled back in time to a few years ago. This is why rocket launchers exist before they were invented. Sparta wanted to sue Master Chief, but he wasn't born yet, so they couldn't sue him. By the time he was born, they had already forgotten.