Shipping costs aren't purely based on the freight/USPS charges. Shipping/warehousing facilities charge "picking" fees for every single item that goes into a box/envelope/etc.
Think of it this way: it takes a worker significantly more time to count (and double check) 33 packs of magic cards and then put them into an envelope/box than it takes to grab 1 factory sealed booster box and put it into packaging. Hence, it's more "costly" to ship 33 booster packs than it is to ship 1 booster box.
Whether you're ordering 3,6,9 or 12 packs of cards, the shipping materials are different sized envelopes/boxes. So when doing fulfillment, they have a handful of different packaging options, also causing complication- versus "1 or 2 booster boxes which fit in Bag A or Box B".
Is the cost 100% purely linear? absolutely not, but it's a reasonably close approximation. There is honestly nothing else we can do about the shipping costs.