Here's why:
* Informal and Contracted: "Ain't" is a contraction of "am not," "are not," "is not," "has not," and "have not." It's considered informal and often seen as grammatically incorrect in formal writing and speech.
* Regional Variation: While "ain't" is widely understood, its use varies depending on region and social context. It's more common in certain dialects and informal settings.
* Prescriptivist vs. Descriptivist: Dictionary compilers often take a prescriptivist approach, focusing on "correct" usage. While "ain't" is used, it's not generally considered a "correct" word. Descriptivists, on the other hand, document how language is actually used, including informal and regional variations.
However:
* Some Dictionaries Include It: Some dictionaries, especially those focused on regional dialects or informal language, may include "ain't" with a note about its usage and informality.
So, while "ain't" is a word people use, it's generally considered informal and not listed in most standard English dictionaries.