Here's why:
* Short vowel sounds are typically pronounced quickly and with a shorter duration. They are often represented by a single vowel letter in spelling.
* Long vowel sounds are usually pronounced longer and often have a more drawn-out sound. They are sometimes spelled with a single vowel letter followed by a silent "e" or a vowel digraph (two vowels together).
In the word "pig," the vowel sound is a short "i" sound, as in the words "fit" or "pin." It's a quick, short sound.