Here's why:
* Short vowel sounds are typically quick and sharp.
* Long vowel sounds are typically drawn out and can often be represented by the vowel itself followed by a silent "e" at the end of a word.
The "i" in "swift" makes a short "i" sound, similar to the "i" in "sit". It is not drawn out like the "i" in "bike" or "time", which are long vowel sounds.