There are other dependencies that React uses outside of browser compatibilities.
Some features that are part of the core features of React such as JSX utilize Babel.

React components are essentially JSX components.

• Remember, JSX is the HTML-like code that we use to build React components.

Because the browser doesn't understand handling this directly (browsers do quite a bit these days), React makes use of Babel to compile the JSX into the Javascript and HTML that the browser can understand.

This of course isn't the only reason, but is a critical example of why Babel is necessary.