I can’t speak to the decisions on the parts of whoever is building the frameworks. But I will say the dev experience working on a frontend app using React, Vue, etc. is substantially greater than using something like Django templates. And I say that as someone who has written a great deal of both.

I really wish I could be as productive in Django templates as I am with React, but alas. Once you go to component based UI development it is hard to go back. Especially when you get used to the high quality UI libraries.