Here's why:
* Short vowel sounds are typically quick and sharp. In "fly," the vowel sound is short and quick, similar to the sound in "pit" or "hid."
* Long vowel sounds are typically drawn out and often have a "glide" to them, like in "pie" or "hide."
The "y" at the end of "fly" acts as a consonant, not a vowel, so it doesn't contribute to the vowel sound.