Why is it that tooth cavities don't heal?

Invasion: Once the structure of the tooth is invaded and damaged by the bacteria/acid desolving process, it can not reform on its on. That's why we stress prevention to stop the destruction begins on the tooth.

Non-Organic Structur: The main reason that dental enamel can't heal is that it is a non-organic structure without a blood supply. The inside of teeth generally can't heal due to advancing bacteria which can't be eliminated completely, and also a limited immune response within teeth due to a very low blood supply. Very small cavities can, however, be healed with Fluoride in some cases. Preventing cavities is key.

Can't: Teeth lack the ability to generate new tooth, even if the decay is removed. They can build reparative dentin underneath, but not grow new tooth. Cavities are holes resulting from bacterial destruction. The tooth structure is gone, and the bacteria remain to continue the destruction. Nothing there, even after removal of bacteria, to fill hole back in. Teeth do not continue to grow, like skin.