If you go to Walmart, you don't expect the price labels to say "$0.50 Kellogg bulk unit cost, $0.80 retail storage fee"; you just expect "$1.30", and the store's costs are their own concern.
It would certainly be a nice thing to do, but we generally don't expect retailers to facilitate you buying from their supplier.
On the other hand, retailers also probably have explicit permission from vendors to resell the good, which DoorDash doesn't.