I'm just wondering, if you have a cut, it will heal itself. why can't a tooth cavity heal itself?

Very simple: Your skin is living tissue.. It has cells that can divide and repair damage, like a cut. The enamel and dentin of your tooth is not alive... There are no cells to divide and repair the damage. Only the nerve in the center of the tooth is alive, and if it is traumatized too much, it dies. Your enamel is comprised of an inorganic matrix of calcium and other materials.

Oranges and apples: Skin and connective tissue easily replicates itself for repair. The enamel of teeth is mineral and has no living cells to repair itself.