Because a cashier's check is for a fixed amount; you can't change its value on the spot without going through the whole process again.

Suppose you show up to buy a truck with #30K in cash (the truck is listed for $30K). You inspect the truck, and find that the A/C needs to be fixed, which would cost you, say $3K. So you decide to split the repair cost with the seller, and now the truck will cost you $28.5K. If you have cash, this is simple: you just hand over $28.5K. But if you have a cashier's check?