All other titles, used when addressing someone, have a kanji character. Was/Is there a kanji term for -san?
Totally not the question you wanted, but a piece of trivia nonetheless: while there is no kanji for さん, there is a hànzì, and it's 桑: the Chinese Internet users needed to render the non-kanjified Japanese honorifics phonetically, so you'll find the versions for many others as well. (醬 is ちゃん).
- It actually has a precedence, as originally さん came to Taiwan during the Japanese power and got actively borrowed to Taiwanese Min Nan (Hokkien): and it was there that 桑 was taken as a phonetic. Example, 歐巴桑 おばあさん. Strictly saying, 桑 is used as a phonetic for any occurrence of [saŋ].
It wouldn't work in Japanese, as the on-reading is now そう.