Because I cannot stand writing “mic” (rhymes with “kiss” or “kick”, we’re not sure) when we say “mike” (rhymes with “Mike”).
Also, mic is methyl isocyanate:
Bad news! The Washington Post has abandoned “mike” in favor of “mic”. Unbelievable!
(According to the Washington Post opinion piece, “mic” is the abbreviation used by manufacturers of recording devices to identify the correct jack for the microphone. An abbreviation was usually necessary due to the tiny amount of real estate available. In other words, “mic” never had a phonic or sonic origin.)