If rule-based highlighting of syntax is meaningfully helpful for reading code then how come nobody uses rule-based syntax highlighting of prose?

Why don't we visually emphasize the verb in a sentence and deemphasize articles and prepositions, for example?

In prose the author can *bold* or italicize text to emphasize ad-hoc, or make ## headings for scanning. Why can't we do such ad-hoc emphases in code?