Why is the letter “I” capitalised, when other personal pronouns like you, he, she, and so on are not?
This goes back to the middle ages.
At first English used a variation on the German equivalent of ich, but this was later shortened to i. But a thin and solitary letter easily got lost on a page of text, so it was made upper case to better stand out as a word in itself.
Other languages don’t capitalise the first person personal pronoun. French doesn’t capitalise je, Spanish doesn’t capitalise yo, and German doesn’t capitalise ich, as they aren’t single letters. But in English we capitalise I because it’s easier to read.