Here's why:
* Language families are groups of languages that are thought to have descended from a common ancestor. Examples include Indo-European, Afro-Asiatic, and Sino-Tibetan.
* Languages are distinct systems of communication that are mutually unintelligible. English, Spanish, and Mandarin Chinese are examples of languages.
* Dialects are variations of a language that are mutually intelligible. For example, American English, British English, and Australian English are dialects of English.
Therefore, a language can belong to a language family, but a dialect is simply a variation of a language.
For example, the English language belongs to the Indo-European language family, and its dialects (American English, British English, etc.) are all variations of the same language.