I think it is easy to intuit the reason why "I" is capitalized.
"I" is simply the most important thing in the world or language, so it gets the special status.
The world exists because I exist first to perceive and understand it.
If I don't exist, there would be no world, no God, and no human race of over seven billion.
"I" is the single most important concept, the start point of everything, so to me it seems natural that "I" is capitalized.