Why do we capitalize the word “I”?

Maybe it's symbolic ...

The word “capitalize” comes from “capital,” meaning “head,” and is associated with importance, material wealth, assets and advantages. We have capital cities and capital ideas. We give capital punishment and accrue political, social and financial capital. And then there is capitalism, which is linked to private ownership, markets and investments. These words shore up the towering single letter that signifies us as discrete beings and connote confidence, dominance and the ambition to pull ourselves up by our own bootstraps.

Or not ...

“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.”