Humans have artificially extended lifespans, and also have a diet that contains far more sugar than other species.
The human species, before medical advancements and nutritional awareness, had a lifespan that was typically around 40 years. Humans also had a diet that comprised primarily of meats and vegetation, with no processed food. As a result, the teeth in an adult lasted a sufficient time with no cleaning to allow for a full lifespan.
Nowadays, we consume sugar regularly, which is a breeding ground for bacteria in the mouth. We also live to 70 years or more, well beyond the original rated lifespan of teeth. So we have to keep our teeth clean to make sure they last.