Here's why:
* Short vowel sounds are typically found in words where the vowel is followed by one consonant. In "hill", the vowel "i" is followed by the consonant "l".
* Long vowel sounds are often found in words where the vowel is followed by a vowel or by a consonant and a silent "e".
Since the vowel "i" in "hill" is followed by a single consonant, it makes the short "i" sound.