Here's why:
* Short vowels have a quick, clipped sound. Think of the "a" sound in "cat" or "hat".
* The "a" in "America" has a similar sound, close to the "a" in "father".
While the "a" in "America" isn't pronounced exactly like the "a" in "cat", it's still considered a short vowel because it's a single, quick sound, not a drawn-out one like a long vowel.