Here's why:
* Short vowel sounds are typically represented by a single vowel letter in a word. In "hat", the vowel "a" is a single letter.
* Long vowel sounds are often represented by a vowel letter followed by a consonant, another vowel, or a silent "e" at the end of the word. For example, "hate" has a long "a" sound.
Therefore, the "a" in "hat" makes a short vowel sound, similar to the sound in "cat" or "bat".