It's not as simple as taking a 15 minute drive to the DMV and waiting in line. Many of these people lack access to proper transit systems to get to these places. At least where I live, the DMV is open during normal business hours which may be impossible to get to without taking time off of work. Many of these people are paid hourly which makes this a financial decision as well. It's really hard to miss an hour of work when that could be food or rent.
It's really hard to come up with a solution when large swaths of the country or totally opposed to anything that slightly resembles a national ID. While I agree that we could probably be doing something, it's not as simple as telling everyone to go to the DMV.