It has nothing to do with how good your template language is. It's simply that a controller's main controller (not talking about partials here) has an implicit dependency on the controller, and the controller has a dependency on the template. Unlike, say, data models, the relationship is not unidirectional.

But really, what is this separation for? What does it gain us? We generally separate stuff out into declarations and files because it gains us modularity ("separation of concerns") and reusability. Data models are constantly reused, for example, as are views. But that one template belonging exclusively to the controller isn't reusable. It's separated out for no particular reason other than the fact that, historically, it has been expressed in a different language and must be parsed and "compiled" separately, so it has not been about "concerns" but about implementation details.

React removes that step, and because the template is unequivocally tied to the controller, there is nothing gained by separating them. The point of React is that the "template" is the component. The component's rendering is intimately tied to how its internal state and data model is wired up because the one half wouldn't work without the other, and vice versa.

(As an aside, in a well-designed, pure component, there is little state and the component is mostly template, anyway.)

(As an aside, most template languages don't even let you validate a template's inputs — that is, declaring that it requires specific named inputs and what their types/structure are. No idea why, seems pretty obvious defensive functionality to me. But React does this.)