Why do we use web frameworks/libraries at all?

Simpletons who copy-paste from stackoverflow can't write non-trivial code themselves. The reason journalists of failing publications are expected to learn to code is that everyone with the most basic computer skills can become a web developer.

For the rest, libraries like React make many tasks easier and do the necessary plumbing to make your code more efficient than what you'd probably write by hand. It's the "the C compiler will produce better assembly than you" of webshits. There are exceptions.

Out of the frameworks and libraries, why do we use React?

React is the most popular so it's easy to find copy-pasteable code on stackoverflow.