Here's why:
* Short vowel sounds are typically found in words where the vowel is followed by one consonant.
* Long vowel sounds are usually found in words where the vowel is followed by a consonant and a silent "e", or a vowel combination like "ee" or "ai".
In "cozy", the "o" is followed by the single consonant "z". This makes the "o" sound short, similar to the sound in "cot" or "hot".