Why the personal pronoun I must be always capitalized?
I don't know the reason why I can't write i am but you are, she is, etc.
I got this from the internet on the history of capitalizing "I."
"The pronoun 'I' developed from the unstressed form of Old English (about 725) as a singular pronoun of the first person (nominative case). Modern and Middle English I developed from earlier i in the stressed position. It came to be written with a capital letter thereby making it a distinct word and avoiding misreading handwritten manuscripts. In the northern and midland dialects of England the capitalized form I appeared about 1250.
So it seems that it was at first a stressed word, but to avoid misreading it in handwritten documents it became a capital "I."
Another history online about the capital "I."
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.
Graphically, single letters are a problem, says Charles Bigelow, a type historian and a designer of the Lucida and Wingdings font families. They look like they broke off from a word or got lost or had some other accident. When I shrunk to a single letter, Bigelow explains, one little letter had to represent an important word, but it was too wimpy, graphically speaking, to carry the semantic burden, so the scribes made it bigger, which means taller, which means equivalent to a capital.
The growing I became prevalent in the 13th and 14th centuries
I guess it's a rule to make clear when "I" is meant to be a personal pronoun or just a single letter