Why doesn't a 100-year-old tree have just an epidermis on its surface?

A 100-year-old tree has multiple layers of bark to provide protection and support to the tree as it grows. These layers include the cork cambium, cork cells, and phloem, which help the tree withstand various environmental factors such as pests, diseases, and physical damage. The epidermis alone wouldn't be sufficient to provide the necessary protection for a tree of that age.