Here's why:
* Short vowel sounds are typically quick and sharp. They are usually represented by a single vowel letter in the spelling.
* Long vowel sounds are often longer and smoother. They are often represented by a vowel followed by a silent "e" or a double vowel.
The "i" in "chin" makes a short "i" sound, similar to the sound in "pin" or "tin". This is a short vowel sound.