Until the early 2000s, browsers didn’t have the capabilities they have now. They were a lot less powerful, and building complex applications inside them was not feasible performance-wise. The tooling was not even something that people thought about.
The second wave, which is the current one, has React, Angular, and Vue as its main actors.
Note that jQuery, Ember and the other projects I mentioned are still being heavily used, actively maintained, and millions of websites rely on them.
Frameworks abstract the interaction with the browser and the DOM. Instead of manipulating elements by referencing them in the DOM, we declaratively define and interact with them, at a higher level.
Using a framework is like using the C programming language instead of using the Assembly language to write system programs. It’s like using a computer to write a document instead of using a typewriter. It’s like having a self-driving car instead of driving the car yourself.
Well, not that far, but you get the idea. Instead of using low-level APIs offered by the browser to manipulate elements, and building hugely complex systems to write an application, you use tools built by very smart people that make our life easier.