Your contractor may be right and that's the best they can do.

Because appliance suppliers base discounts on how much one buys, some contractors who have several jobs going at the same time can combine the appliance orders of each client into a single order. So the suppliers are asked to bid on a lot more stuff and they may be more inclined to provide a deeper discount. Whether your contractor passes the savings to you is another story. So just go ahead and ask or suggest this.

The other alternative is to find homeowners who may have separate remodeling contractors and have the homeowners approach suppliers as a group. Not sure if anyone has tried this though.