Why do they call marines bullet catchers?

Answer 1

First in, last out. They may not be very smart but they're good at lifting heavy things.

Answer 2

The term "bullet catchers" is sometimes used to describe Marines because they are often on the front lines of combat situations and face a higher risk of being targeted by enemy gunfire. The phrase may also highlight the courage and bravery required to serve as a Marine, as they willingly put themselves in harm's way to protect others and accomplish their mission.