Because it’s terrible and can go against you without your influence
If someone blasts it at you and your arm is by your side it should be a penalty because your arm exists? You can’t get the full thing behind your back or potentially react if running etc.
This black and white if it hits the arm it’s a foul is garbage (and also not equally enforced as we’ve already seen)
Life isn't fair. Just because you didn't mean to foul someone doesn't stop it from being a foul so why are handballs any different. I agree that they should be equally enforced but if that doesn't hit his arm thats probably not a goal.