It would be great if React could be built directly into browsers, but it would greatly curtail the current flexibility of server-vended React. The project is able to evolve quite quickly unshackled from a w3c process and the pulse of major browser updates.

(IIUC, there was a proposal in Firefox decades ago to make the engine into several flexible modules and a page could declare which modules it depended upon, then the browser would either cache them and use them for multiple sites or already have them builtin. You'd get the best of both worlds: rich and expressive pages without the frequently-paid cost of poly-filling the gap between how the developer wants the render engine to work and the actual implementation of the render engine.

Sadly, I suspect the actual complexity to implement would have made for a worse overall situation than what we have now).