Their teeth keep growing because that's how they evolved. Rodents are very rough on their teeth. They eat nuts with hard shells, they chew through walls to get at human food, and larger rodents like beavers chew through trees. If their teeth didn't keep growing, they'd gnaw them down to nothing and would starve to death. This is similar to why animals like sharks have multiple rows of teeth. They loose teeth so often that if they didn't have extras they'd starve.