Here's why:
* The vowel sound in "chain" is short. It's the same sound as the "a" in "cat" or "man."
* Long vowels are typically represented by a vowel followed by a consonant and another vowel (e.g., "ate," "hope") or by a vowel followed by a silent "e" (e.g., "name," "bike").
The "ai" in "chain" doesn't follow this pattern. It creates a unique short vowel sound.