Simply because Rails is not designed to solve frontend problem where complex user interactions and client side state are included, not to mention partial rendering and transition. Rails is a backend framework after all, spitting a HTML is all it can do in terms of a frontend part. Just don’t know why you think you can solve all the modern frontend user interaction with a backend framework like rails and Django,,,,,,,,,,,,,,
Github, Hey, and so many others are examples of Rails applications with complex interactions. Server templates and partials rendering doesn't mean "I reload the page on every click and can't write any javascript because I only do ruby". You have tools for doing dynamic html too, just not your usual redux/rxjs/thunk/observable/typesafeactions stuff.
Agree that Django is a backend framework, but Rails is not, you can certainly do frontend in Rails. Just not SPAs, but you need a more open mindset for understanding how it is intended to be used.
Of course, if you are building Google Docs or Maps then you're probably better with a SPA framework. But not for 90% of applications out there which are just CRUD apps.
