> I have programmed in Servlets, JSP Model 2, ASP.NET, iOS, Android, Windows. None of these require a state management library.

All of these have State Management involved somewhere, you might not "obviously" have a "State Management Library" because your stack might encode it simply as "best practices" or it might be a low level detail in the stack that you treat as a simple, black box, but more often than not in many of those environments you are building ad hoc state management libraries all over the place, each application its own little snowflake of state management.

As someone else pointed out, you might be strongly relying on a database or an ORM as your primary state management library. Most React projects are using In Memory data structures, for a vast majority of their work, and most state management libraries you can think of as simply an "ORM-like" thing for managing CRUD in memory.

But we can push the abstraction further. The ORM is often used to build the "M" in an MVC pattern. There are all sorts of complicated Model patterns in various MVC frameworks. Some have more boilerplate than others, some have complex engines and libraries that do a lot of management work. A lot of models use "plain old class objects" (POCOs) of one sort or another, but then have all sorts of ravioli code inside those classes to maintain that the state follows business logic rules, and notifies other classes of necessary changes, etc. That of course depends somewhat on the separation of concerns between your Models and Controllers. (I've seen a lot of "MVC frameworks" and "MVC apps", and almost no two are alike in their M/C separation choices, or their overall state management architecture.)

React itself solely concerns itself with View as a framework; it leaves Models/Controllers entirely to the user. Various React state management libraries are different variations on Model/Controller logic/preferences.

(Some React state management libraries more explicitly define the Model/Controller separation and powers than just about any MVC framework for the platforms listed above.)

As you get into more recent, more "reactive", patterns you see a lot of movement toward "MVVM" Model-View-ViewModel. In those patterns Views and ViewModels are a bit more tightly coupled than MVC. Often a VM has a way to signal data changes (for "two-way" binding) to the View so it can update just the things that changed. In .NET the pattern is called INotifyPropertyChanged. Most other JS frameworks right now (Angular, Vue, Aurelia) follow similar patterns to MVVM, somewhat tightly coupling Views to ViewModels in a similar way to MVVM, even if the frameworks often obscure those details in proxy/signal "automagic". If you look at MVVM projects you will also find some variety for how data changes are signaled. Even .NET's INotifyPropertyChanged has lead to a variety of different toolkits and MVVM libraries designed to manage it and orchestrate it.

React instead only "allows" one-way data binding in views. So even from an MVVM standpoint as opposed to an MVC standpoint, this leaves React state management libraries with obvious goals to do some of the INotifyPropertyChanged-type actions of knowing which state changes at a time, giving React a leg-up in its "one-way" data binding to better know when to re-render components based on state changes. (In many cases a benefit here is that most React state management libraries don't necessarily return to tightly-coupling Views and ViewModel equivalents, most of them look more like MVC than MVVM, despite something of a closer comparison to MVVM in desired outcomes.)

Most of the individual state management concepts in React state management libraries exist in other frameworks, they just often have other names (ORMs, Model Libraries, Controller Patterns, MVVM Frameworks, etc) or are even considered as "user" parts of the stack, left to the individual project to build ad hoc as needed.

Part of what React's state management diaspora has been good for is questioning some basic assumptions in MVC and MVVM patterns. One of those assumptions is "mutable state" where you might just directly update POCO objects each time state changes. (Which works, and has been used for a long time, but also has a history of interesting bugs and debugging issues.) The library shown here is interesting because it's one trying to get the programming benefits of immutable state with the "POCO" terseness of mutable state (`draftState.itemList[55].name = 'new name'` instead of something more like `nextState = curState.setIn(['itemList', 55, 'name'], 'new name')`).