A confirmation (creating a block) is an expensive operation. However, every operation can in principle be performed by any computer. Suppose that a seller waits for 6 confirmations for a transaction T0. After the seller receives confirmations and gives goods, the buyer inserts into the network 7 prepared confirmations confirming a conflicting transaction T1, and T1 expels T0. Money disappears from seller's account.
If the buyer spends 30 days in preparing 7 confirmations and the network spends (1/6/24) days producing every confirmation, than the buyer should own only (1/6/24)/(30/7) = 1/617 part of the network -- not so much. If this fraud is possible, should not seller's behavior (how many confirmations to await) depend on the amount of transferred money?