eBay will tell you it is to discourage sellers from having a low item price and a huge postage price to avoid Final Value Fees.

They will also tell you it is to have a level playing field for sellers who factor shipping into their item price and offer "free shipping".

Some years back eBay were pushing hard for more sellers to offer "free shipping". They quoted stats that clearly showed that sellers who offered free shipping sold more items. They didn't give any details about how they came to that conclusion, but eBay said it so it must be true, eh?

They also pushed for more Buy It Now listings. Having more big store sellers reinforced this push because these stores don't do auctions.