Well for certain cultures 1 and I were literally the same. That same culture also I believe didn't have lower case. Some brief searching brings me to some art

Well for certain cultures 1 and I were literally the same. That same culture also I believe didn't have lower case. Some brief searching brings me to some articles that show Latin lowercase came from monks/scholars under early French King Charlemagne. I guess if you're a monk manually recopying texts, simpler forms are better, even if some forms look like other forms. And it's definitely simpler/quicker to rout out a single stroke for I rather than give it the top and bottom lines.