Why is the letter/word I capitalized but the letter/word a isn't?

Well, I refers to yourself a person. A refers to a thing. So unless a is at the beginning of a sentence, It won't be capitalized.

Good luck! Have a Great day!

"I" is in regards to yourself, you and any humans name are proper nouns that you go by. "a" is not a proper noun it is describing the quantity or the subject of the matter, Ex - [I] love it when [a] cool summer day takes place in my area.

{Bad sentence, sorry}