Why? You don't think they wanted to serve their country, too? I'm sure that many of those men were also looking to better themselves, and they thought that being part of the military would help them do that, just like many other young men from less than wealthy circumstances all across the country.
I have no doubt that they did and thirteen won the MOH. My point was the number of 500,000 is much too high given their percentage of the population at the time.