Because certain Sketch properties—like z-index—aren’t named, they can't be mapped to the design system in the same way DSM maps text styles and layer styles.
Additionally, layers in Sketch documents don’t include a unique ID like symbols do. Therefore, Inspect can only identify symbols in a document that are present in DSM. It won't detect non-symbol layers.
It's best practice to use symbols in a Sketch UI tool kit, as symbols allow you to easily propagate changes and define overrides, which set the foundation for a maintainable design system.