Why are teeth not able to heal by themselves?

No blood supply: Enamel has no blood supply and therefore is unable to heal itself detin does heal to some extent.

Developmental reason: Teeth develop in multiple layers. The cells that lay down enamel- enamelblasts stay outside the enamel after its formation and are subsequently lost or exfoliated . Thus the enamel does not have the power to heal itself. The second layer dentin has power to heal itself rather slowly and does so by forming secondary dentin.