Because 4 or 5 years of real world React development have proven them inadequate suggestions.

React itself was created in opposition to arbitrary separation of concerns dogma. The concern is the component, the entity encompassing both UI and data manipulation code, as well as styling. Separating all of these parts into isolation does not result in more maintainable code or better UX.

No, React was created in opposition to the fact that we used to manually update the DOM because that's all we had tools for.

Rethinking best practices absolutely addressed misapplication of separation of concerns.

https://www.youtube.com/watch?v=x7cQ3mrcKaY

Components have always had JSX markup along with data manipulation logic. CSS in JS and styling co-located with component extended this consolidation of cross-cutting concerns.

These, along with not manually updating DOM state as you mention, all fall under declarative aspects of React.. which concurrent rendering and Suspense are consistent with and essentially an extension of.

No, React was created to allow contractors to write code fast and not worry about optimization.

React isnt the one mandating how its used. It can be used in the way that the GP is talking about, but you can also keep the data entirely separate. Redux et al do exactly that. The main thing that the GP is missing is that, like with video games, in order to keep the view rendering smoothly, you need something like double buffering so that if the render isnt complete, you dont block. That is independent of how the data is being provided.