There are two reasons for the factory height;
The magazine height as explained before, as well add a VZ Chest rig and 8-10 mags on your front and you will find that you will be propped up to that height when firing prone.
The bipod is not used much in the service and was never intended to be used for sustained fire. In the 50's when this rifle was made the average soldier was still trained to fight from trenches so having something to rest your rifle on made sense.