I'm not a dentist . But I do know that there is not a mechanism whereby decayed tooth material is extruded. Enamel that is eroded does not replace itself. Dead nerves inside of teeth don't regrow. God didn't design teeth to do these things - but i think that is why he made dentists.
Nature. Like many parts of the body, teeth are not different. When a tooth gets decayed, there is no mechanism in the body to seal it off and repair it over time. It is unfortunate, but that is why dentistry came into being.