Why are the Lego boxes so unnecessary big?
The possible reasons include:


Surface area needed for legal info, practical requirements like barcodes, information, feature descriptions and artwork
Aspect ratio of the artwork means the boxes can't be too long and thin
Leaving space between the bricks so they don't get squished up against each other
Size of the instruction manual or sticker sheet
Psychological tool to increase the perceived value of the set
Larger boxes grab the attention of the consumer better
Larger packages are harder to steal


But the exact reason is likely a combination of some of the above ones and maybe some others, most probably in a top-secret ratio known only by LEGO.