You're right that immediate mode is great and makes life so much easier but for most use cases, React is very much a "retained mode" framework. If all of the valuable state in your app has its source of truth on the server (majority of apps), then retaining any of it in React state soup is unnecessary complexity.

People like to bring up how terrible it was to do vanilla.js in the old days but completely ignore that we are not, in fact, still in the old days. Native browser APIs are so much more powerful now and a ton of the pain that React was made to solve is even now handled declaratively with some simple HTML and CSS. No hacked out JS required.