Here's why:
* The vowel sound in "good" is a long "oo" sound. This is the same vowel sound as in words like "moon" and "food".
* Short vowel sounds are typically found in words like "cat", "bed", "dog", etc. These words have a shorter, more clipped vowel sound.
So, while "good" is a short word, it doesn't have a short vowel sound.