Why We Need Matching Methods

Why couldn't Angrist just compare the lifetime earnings of veterans and nonveterans with standard regression methods?

50 XP

Because the people who applied to the military and got rejected are probably poor people in the first place.
Because the military only rejects disabled people.
Because the veterans who selected into the military may have had very different earnings trajectories than individuals who had not selected into the military.
Because people who got rejected are likely to be disproportionately female.