Here's why:
* Short vowel sounds are typically quick and sharp. In "feet", the vowel sound is short and clipped.
* Long vowel sounds are typically drawn out and often have the same sound as the vowel's name. In "feet", the vowel sound is not the same as the name of the letter "e".
The vowel sound in "feet" is the same as the vowel sound in words like "bet", "met", and "set". These are all short vowel sounds.