Because there are a significant amount of people who don't have any. Seriously, there are a lot of elderly people who simply have no need for a driver's license

Because there are a significant amount of people who don't have any. Seriously, there are a lot of elderly people who simply have no need for a driver's license, so they don't have one. And if they're old enough, it's possible their birth certificate is not easy to get, or may even require traveling across state lines to do so. Where's the money for that trip going to come from? And for people who work, they're not always able to just take a day off to go to the DMV. And the DMV is only open during... working hours.