The menu bar is essential for operation of the client. Though it's possible to memorize shortcuts for common actions, it's dangerous to offer the ability to hide the menu bar -- there is no way to unhide it without using a shortcut, and a user that does this accidentally or without realizing what they're doing without knowing the shortcut to unhide the menubar would be permanently locked out of using the application.

On Mac and some Linux desktop environments, the menubar is always shown at the top of the screen, so there's no way to hide it on those platforms.

On Windows, you can enable the "custom compact window borders" option, and then the menubar will share space with the titlebar, which is better than hiding the menubar and saves the same amount of space.