Here's why:
* The "o" in "window" has a short vowel sound. It sounds like the "o" in "hot" or "lot".
* Long vowel sounds are usually associated with a vowel letter followed by a silent "e" at the end of a word (like "make", "hope", "kite") or with a vowel digraph (like "rain", "boat", "tree").
Therefore, the vowel sound in "window" is a short vowel sound, not a long one.