title: Why running index.html won't work for a `create-react-app` build?
body: <p>I am new to <code>create-react-app</code>.
  I want to know why opening the <code>index.html</code> form a build does not work ??
  I know to serve a build <code>serve -s build</code> should be used, but I want to know why a build react app can’t be started without serving.</p>
  
  <p>I will explain further:
  What I do is...</p>
  
  <ol>
  <li><code>create-react-app helloworld</code></li>
  <li>Make some code changes and verify that app is running ok.</li>
  <li><code>npm run build</code> or <code>yarn run build</code> ... Now I will have my <code>./build</code> directory created with <code>index.html</code> in it.</li>
  <li>Now I just open <code>index.html</code> in browser and it won't work.</li>
  </ol>
  
  <p>I want to understand why this does not work? 
  Why I have to serve the build to make it work?</p>