Because it's expired. Here in Michigan you're required by law to renew your driver's license every 4 years on your birthday. Not renewing it in time, would mean you're not allowed to drive your vehicle until you're licensed again. Sure you could carry around your expired license, but they won't accept it if you happen to be stopped by police or intend to go to a local bar for a drink.
If it's just a basic state ID, then you might be okay to hold onto it, but a regular state ID doesn't get you driving privileges. You still need to renew an standard ID every 4 years though. At least in Michigan.