To use memoization you must use immutable structures (otherwise React is unable to detect changes) which are a pain to use and are slow if not designed properly. Do not believe me, try designing for example an algorithm that accepts a huge immutable sorted list with keys, a large sorted list of bulk deletes/inserts and produces a new sorted list with minimum operations. I made 2 or 3 prototypes and they still are not good and not even working properly.

In my book, a good UI framework simply attaches to a mutable model made of arbitrary objects, detect changes and displays it with minimum number of operations. Doesn't require to use any proxies or immutable data or store data in components. Shouldn't be difficult. Why nobody made it, I do not understand.

React isn't immediate mode by my understanding.

In my head it's retained mode + the diffing layer, which sounds the same but if you try to write an "immediate-mode" API (say, using hooks) in React you'll find yourself churning a lot.

At least unless you use the magic React compiler that memo-izes everything correctly.