You asked a good question. There are different explanations. It probably has something to do with the fact the personal pronoun "I" is a distinct word, so it has evolved into a capital letter to avoid confusion.

in my opinion,"I" represent a person ,in the Arabic numberals,1 represent a person.maybe there is some conntants

I have an idea from the famous book explaning about this idea you suggested. English, including western culture, have a tendency to propose, intensify their NOUN. Contrary to that of eastern of which people usually don't care to use NOUN, but more likely to use VERB in their context. Furthermore, "I" is absolutely the most basic one than that of others so I think it's the answer of your question.