The only other one-letter word is “a”. And that’s not even a “real” word, but an article. I think that’s because “I” has to be the most used word in the English vocabulary, so it’s better kept short & precise. Plus it’s always written in capital. All sounds a bit egoistic doesn’t it?
~Ugh, this irresistible cream cheese spread is undoing all my hard work at the gym!~