Because Rails isn't good at building modern frontend experiences. It's built around static pages.
RSCs (and Next) are (going to be) built for dynamic pages. Loading new data from the backend is as simple as editing an ERB template, and you get updates on top of that. Suddenly you don't need to think about APIs nearly as much, you don't need to worry about making fetch requests, you don't need separate codebases.
If Rails decides to make data and UI streaming a primary building block then that would be awesome. Until then, that's why you would use RSCs.
RSCs (and Next) are (going to be) built for dynamic pages. Loading new data from the backend is as simple as editing an ERB template, and you get updates on top of that. Suddenly you don't need to think about APIs nearly as much, you don't need to worry about making fetch requests, you don't need separate codebases.
If Rails decides to make data and UI streaming a primary building block then that would be awesome. Until then, that's why you would use RSCs.