In “Me, Myself, and I,” Caroline Winter explains why we capitalize I and not me:
England is where the capital “I” first reared its dotless head. In Old and Middle English, when “I” was still “ic,” “ich” or some variation thereof—before phonetic changes in the spoken language led to a stripped-down written form—the first-person pronoun was not majuscule in most cases. The generally accepted linguistic explanation for the capital “I” is that it could not stand alone, uncapitalized, as a single letter, which allows for the possibility that early manuscripts and typography played a major role in shaping the national character of English-speaking countries.
Hat tip @grammargirl.