Why are typefaces classified by the style of their letters rather than only by their date of creation?

Typefaces are classified by the style of their letters rather than solely by their date of creation because their design is fundamentally rooted in the evolution of historical manuscript models rather than a linear progression of time.1

Early typography was explicitly designed to mechanically reproduce the scripts used by scribes before the invention of printing. Because these letter forms—such as roman, italic, and black letter—originated from diverse calligraphic traditions, their aesthetic characteristics are more descriptive of their lineage than their production date.1

Furthermore, modern designers frequently create revivals of historical faces. These contemporary reworkings, such as those categorized as "old style," are distinct new creations that share stylistic affinities with older models but differ in execution. Consequently, classification systems focus on the physical attributes of the letters—such as stroke contrast, stress, and serif shape—to provide a more accurate and useful framework for bibliographical and technical identification than a simple chronological list.1