Because SMS is horribly limited. 140 chars per message* (less if chars are not plain vanilla ASCII), no support for attachments, group messages, reliable delivery receipts, emoji reactions, etc etc.
* There's a terrible hack called concatenated SMS that strings together multiple messages to build one longer message under the hood, but if any of those parts go missing along the way, the whole thing gets dropped on the floor.
* There's a terrible hack called concatenated SMS that strings together multiple messages to build one longer message under the hood, but if any of those parts go missing along the way, the whole thing gets dropped on the floor.