That area is the "Page Cover". It can be turned off completely in Page Builder, but in some cases this may be problematic as it may contain action buttons as well as media uploaded with a post or profile. Generally I see that it as a design flaw as we essentially create a special case limitation which requires to either have some image there or put up with wasted space, as you duly noted.

So, in our new template, and gradually in all other templates we will be shifting action buttons away from the cover area, and also moving the on-page media into a block rather than page cover. This will allow us to make Covers entirely "decorative" and easy to turn off anywhere. Working on it now :)