Here's why:
* The "i" is followed by a silent "gh". Silent letters don't affect the pronunciation of the vowel.
* The "gh" does not create a consonant blend. A consonant blend is when two or more consonants are pronounced together, like "bl" in "black" or "st" in "stop". In "higher", the "gh" is silent, so it doesn't blend with the "h".
Therefore, the "i" in "higher" has the long "i" sound, as in "bike" or "time".