I agree with George that patriotism played a large role here - at least in the early years of the war. Keep in mind that, in order to get a substitute, usually you had to hire him. Not very many men could afford that. In fact, in addition to patriotism, one of the factors in deciding to volunteer was that soldiers were to be paid - not something most farm boys were used to. Also keep in mind that the provision for substitutes was eventually done away with (end of '63?) so was not an option afterward.