Here's why:
* The vowel sound in "fox" is /ɑ/ (as in "cot" or "hot"). This is a short vowel sound.
* Long vowel sounds are typically represented by a vowel followed by a silent "e" at the end of a word (like "bike" or "cone") or by a double vowel (like "rain" or "boat").
So, while the "o" in "fox" might look like it could be a long vowel, it's actually a short vowel sound.