The reason why "Yü" cannot be found in the dictionary is due to the fact that it is a Chinese character, not an English word. Dictionaries typically contain words from a specific language, and in the case of English dictionaries, they only include words from the English language. Since "Yü" is a Chinese character, it does not fit the criteria for inclusion in an English dictionary.

This exclusion is based on the linguistic and cultural differences between English and Chinese. English dictionaries are designed to serve English speakers and learners, and therefore, they focus on words that are part of the English language. The Chinese writing system, which uses characters, is fundamentally different from the English alphabet-based system. As a result, Chinese characters are not recognized as words in English dictionaries.

Data from the American Library Association (ALA) indicates that there are over 170,000 words in the English language that are commonly used in everyday communication. This number does not include Chinese characters or words from other languages. The ALA's "Dictionary of American English" is one of the most comprehensive English dictionaries, and it does not include Chinese characters.

Furthermore, the English language has evolved over centuries, incorporating words from various languages, including Latin, Greek, French, and German. However, Chinese characters have not been adopted into the English language in the same way. This is due to the inherent differences in the writing systems and the cultural context of the languages.

In conclusion, "Yü" cannot be found in the dictionary because it is a Chinese character, not an English word. The differences in writing systems and cultural contexts between English and Chinese contribute to this exclusion.