Elephants only use about four of their teeth at a time, not counting their tusks. If tusks were to be included (since they’re really only upper incisors), an elephant uses six teeth to chew its food. Even though both the upper and lower jaws contain twelve teeth, they aren’t all worn down at the same time. The further back a tooth, the less it’s used.
This allows the teeth to progress to the front of the mouth to wear down, and always ensures that the elephant has fresh teeth when the ones in front wear out. This is much different than how a human’s teeth grow, and rightfully so. Elephants constantly eat, whereas humans eat a few times a day, including the snacks.