If the argument is "what makes ERB / JST / some other templating language better than React for server-side rendering" then sure, there's not really much differ

If the argument is "what makes ERB / JST / some other templating language better than React for server-side rendering" then sure, there's not really much difference. What I suspect was the intent of the original question was "why does a simple app need to be an SPA with an API on the backend", which I do feel is something that adds significant complexity to any project. It's sometimes worth it, but I'll never be convinced that having to maintain an API alongside client-side rendering is faster or easier than a traditional server-rendered application.