Here's why:
* The vowel "a" in "ball" is followed by a consonant ("ll"). This is a key indicator of a short vowel sound.
* The "a" in "ball" has a short, quick sound, not a long, drawn-out sound. This is typical of short vowels.
You can contrast this with words like "bale" or "bail," where the "a" has a long vowel sound.