Here's why:
* Short vowel sounds are typically represented by a single vowel letter in a word. In "sing", the vowel sound is represented by the letter "i".
* Long vowel sounds often involve a vowel digraph (two vowels together) or a vowel followed by a silent "e".
While "sing" has the letter "i", it doesn't have a long vowel sound. The "i" sound in "sing" is a short sound, similar to the "i" in "pin" or "sit".