RS-232 to I2C communication
I2C is half-duplex. RS-232 is full-duplex.

I2C has addresses, RS-232 does not.

I2C specifically writes/reads registers at the destination. RS-232 just send arbitrary bytes.

They are completely different protocols used for completely different things. There is no set relationship between the two and so there is no way to generic way convert between the two. Whatever device sits in between needs to know the context of the messages being sent and received since it must read and re-interpet the messages.

It's like asking why can't I find something that automatically converts books into movies? You just can't run it through a set process. You need someone who knows both books and movies to sit down, go through the book, re-interpret it coherently, then make the movie.

Or it's like trying to translate cultural references in media works. You can't just run them through a set algorithm. You need something in between that understands both languages (understands both protocols) and understands both cultures (knows the context) to do the translation. Hell, it's tough enough to translate cultural references even when you don't have to worry about the language.