Why are "poetry" and "Confucianism" more formal words? This knowledge really puzzles me.

"Poetry" is a supplementary word, the latter word supplements the previous word, and "piece" supplements the unit of "poetry". Similarly, there are boats, people, sheep, flowers and cows in the room.

"Confucianism" is a formal word, and home is the central word. Confucianism modifies and restricts "home" and is equivalent to an expert in writing. The front can be replaced with "novelist, orator, linguist ..."