Here's why:
* The vowel sound is "uh" as in "cup". This is a short vowel sound.
* The vowel is followed by a consonant. In this case, the consonant is "s". This typically indicates a short vowel sound in English.
Therefore, the "ou" in "cousin" does not make a long "ou" sound like in "house" or "out", but rather a short "uh" sound.