Here's why:
* The vowel sound in "hike" is /ɪ/ (as in "bit" or "kit"). This is a short vowel sound.
* Long vowel sounds are typically spelled with a vowel followed by a consonant and a silent "e" at the end (like "hike" would be if it was "hike"). Since "hike" does not have this spelling pattern, it's a short vowel sound.