Here's why:
* Pronouns are words that replace nouns. They typically have multiple letters to convey their specific meaning.
* English has a few short pronouns: "I", "me", "he", "she", "it", "we", "us", "they", "them", "you", and "your".
* One-letter combinations are not considered words in English. They might be abbreviations or symbols, but not pronouns.
Let me know if you have other questions about pronouns or English grammar!