Because that is part of the definition of areas and volumes. You can't have a volume in only one or two dimensions, just as you can't have area in only one dimension.
In order to understand that, lets simplify. Area is 2 dimentional, width and height, while volume is 3 dimentional, width, height, and depth.
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Whenever you are answering volume questions, it's always units cubed (^3).
Density = Mass/VolumeVolume is not cubed, although the units in which volume is expressedmay be cubed units.
7,238.23 units cubed.
units cubed
1331 cubed units