Because we are not used to the idea of teeth healing and are used to drilling and filling it seems implausible.
However, teeth are designed to remineralize and heal.
Of course, they are!
You only have one set of adult teeth and they need to last a lifetime. They experience continual wear and tear and continual teeth healing.
Why would a system as complex and well adapted as the human body not have an answer for this?
Photo by Isaiah McClean on Unsplash