Opinions aside the fact is, it's fee avoidance.

When a buyer gives a star rating of 1 or 2 to the question:

"How reasonable were the postage and handling charges ?" a box appears showing

'Why weren’t you satisfied  with the postage and handling charges ? Did the Seller"

5 options are there to choose from  with one of them being

"Combine multiple items into one box but charge for postage for each item  individually"