Why is no single body feature used to define every phylum or class?

In biological taxonomy, there is no single body feature used to define every phylum or class because relying on a single diagnostic character would result in an unnatural classification system.1

Taxonomists aim to recognize natural groups based on their overall resemblances and evolutionary relationships rather than arbitrary traits. A feature that serves as a defining characteristic in one phylum may vary significantly among closely related members of a class or order in another. For example, while the complex carbohydrate cellulose is characteristic of two plant kingdoms, it appears in animals only within a single subphylum of one phylum.1

If scientists were to force a system where, for instance, all phyla were defined by skeletal structure and all classes by respiratory organs, the resulting hierarchy would fail to reflect the true diversity and evolutionary history of organisms. Instead, taxonomists evaluate a wide range of anatomical, physiological, and genetic characters, weighting them based on their utility in indicating lines of descent and natural affinities.12