The tautological answer is that it's because we the people have made laws that in certain specific situations make people criminally responsible for how the product of their labor is being used.
The practical answer is that it's because we do want to discourage criminals from "splitting liability" by having most of a gang doing some illegal goal together stay "clean" and only delegating a single "fall guy" for the final touch; so criminal law is explicitly written to consider everyone who knowingly assists a crime to be partly liable as well.
The practical answer is that it's because we do want to discourage criminals from "splitting liability" by having most of a gang doing some illegal goal together stay "clean" and only delegating a single "fall guy" for the final touch; so criminal law is explicitly written to consider everyone who knowingly assists a crime to be partly liable as well.