Text messages technically can be split, but why would anyone want to send a message exceeding 200KB? 200KB even in a language like Chinese is a about 15 A4 (letter) pages of text. In Russian or Ukrainian it's about 30 pages, 60 in English. Why would anyone want to send 30 pages as an IM message?
Messages with attachments do not have this problem: attachments, like images or videos are uploaded to the server and sent as links.