Here's why:
* Short vowel sounds are typically found in words where the vowel is followed by one consonant. In "back", the vowel "a" is followed by the consonant "ck".
* Long vowel sounds are usually found in words where the vowel is followed by a vowel or by a consonant and a silent "e" at the end.
Therefore, the "a" in "back" makes a short "a" sound, similar to the sound in "cat".