Custom Theme, Editor won’t wrap text (i.e. change width)

This is normal behaviour. First, the class second_column of your site is set to max-width:100%. This means it has no fixed width. You won’t notice this on a pc screen, but if you reduce you browsers width to mobile size, you will see the text column becoming narrower. This is good, because it means that your site adapts to mobile screens.

However, it also means that there is no such thing as “the width of the text area of my website” and your text editor can’t adapt to that.

I don’t know if you already use add_editor_style to make editor use the same css as on your site. That’s handy for fonts, colours and so on. But if you include the width statement, that still would refer to the width of the editor frame, which normally is fixed. However, with the following code in your functions.php you can change that width:

function wpse236313_width_text_editor() {
  echo '<style type="text/css">
    .wp-editor-container textarea.wp-editor-area,
    .wp-editor-container #content_ifr {
    max-width: 620px;
    border-right: 1px solid #ddd;
    }
  </style>';
  }
add_action('admin_head', 'wpse236313_width_text_editor');

Note that this still means you get different text wrapping on smaller screens than shown in the visual editor.