What you see on that image is the full-screen mode which is the default mode in the block editor (that applies to Pages, Posts and custom post types), and which is intended to help users to work without distraction.
Secondly, there’s also a note in wp-admin/admin-header.php which says that // Default to is-fullscreen-mode to avoid jumps in the UI. — is-fullscreen-mode is a CSS class added to the <body> tag indicating a full-screen mode. Therefore, the default mode should not be changed (programmatically).
However, once the page has been loaded, you can turn off the full-screen mode by simply clicking on the three-dot button (which shows “More tools & options” when mouse-overed) on the top-right corner of the screen, and from the menu that appears, just click on the “Fullscreen mode” option:
Hope that helps! 🙂