Templating engines are far less powerful and extensible than client side frameworks.
FWIW, there are many JS templating engines as well that work with NodeJS too. There are pretty obvious downsides to templating engines that libraries like React and Vue improve upon.
Using flat files as a backend database is great, but not scalable. Does everything need to scale, not always, but it's a lot nicer to have that option and to build it that way from the beginning. It's also ridiculously easy to do so today.
You'd probably be blown away how quickly someone can get a full stack website set up complete with user authentication/accounts management (including reset flows, magic links etc), database integration, front end system, CMS, component design system, and any other goodies.
All this is easily doable in relatively short amounts of time, and makes working with other teams - designers, product owners, marketing teams - so much easier.
Not to mention being able to do everything in a single language. That's a huge win. Engineers can hop around easily and have access to the same dependency management system, language features, etc.