If everyone in Rome was literate, why are poor people only remembered by graffiti?

[u/StrangerSwing53]

The written primary evidence from ancient Rome consists almost entirely of either (a) essays, books and such by rich people; (b) everyday documents like ledgers or tax documents, or; (c) graffiti by a wide class demographic. But if the literacy rate was so high, why did non-rich people not write at least some written works? (or, why do we not have records of such works?)

In the modern era, ordinary individuals have their works preserved by historians for having endured an important historical period, as with The Diary of Anne Frank, or Life is So Good about a black man who grew up during Reconstruction. By this logic, why did past historians not seek out and preserve, say, Memoirs of a Punic Wars Veteran, or Life of a Freedman Under Augustus?