Here's why:
* The "i" in "dim" is followed by a single consonant ("m"). This is a typical pattern for short vowel sounds.
* The sound of "dim" is a short, sharp sound, not a long, drawn-out sound.
Compare it to words like "dime" or "time," where the "i" is followed by two consonants ("me" or "me") and has a long vowel sound.