Most rodents have continuously growing teeth. Like fingernails. In the wild, they constantly gnaw on stuff to sharpen and wear down those teeth.
As pets, if they are fed only soft food, and aren’t given something tough to gnaw on, the teeth won’t wear, and will just keep growing until the animal can no longer close their mouths.
This can be addressed by a visit to the vet, who will grind down the teeth, but you can only get to this point by seriusly neglecting your pet.