Here's why:
* Short vowel sounds are typically quick and sharp, like the sound of "a" in "cat".
* Long vowel sounds are typically drawn out and can often be spelled with a vowel followed by a consonant and a silent "e" at the end, like the sound of "a" in "cake".
The "o" in "sofa" makes a short vowel sound, similar to the "o" in "hot" or "hop".