React components implement a render() method that takes input data and returns what to display. This example uses an XML-like syntax called JSX. Input data that is passed into the component can be accessed by render() via this.props.
In other words, it was more simple than we separate html and render it on React, as a component.
If we still want to access html page via any page that created with react, we can did it via a href.
For any other way like import it (maybe), i think it was too complicated because of some reason.
Hope you will more understand about react with this react docs link
React is used to build
index.html located in the public folder (if you're using create-react-app)