That's the wrong question. That's like asking why ASP.NET is so much better than coding pure C# and handling all of the HTTP-stack and template parsing and such yourself. Using React is better from a developers point of view. It makes certain abstractions and decisions for you. And that's what makes it better.
I decided to not use a JS-framework in my last project because I was on a deadline and figured that I didn't have time to learn something new... Now I'm in jQuery hell :/ I would almost certainly have gotten a better result learning, and using, a framework like React/Reflux/dux/lux/*ux or Angular or Aurelia.