Why do people refer to illegal aliens as Undocumented?

As far as I know, no. Those codes cover issues like employing undocumented immigrants, importing undocumented immigrants, forgery of documents by an undocumented immigrant, failing to depart the country once you have been deported as an undocumented immigrant (all crimes) but to the best of my knowledge there is no code against simply being an undocumented immigrant.

§ 1324. Bringing in and harboring certain aliens

§ 1324a. Unlawful employment of aliens

§ 1324b. Unfair immigration-related employment practices

§ 1324c. Penalties for document fraud

§ 1324d. Civil penalties for failure to depart