Because their reputation is what suffers. Customers think the restaurant is partnered with DoorDash or whoever because why wouldn't they. All the restaurant's

Because their reputation is what suffers. Customers think the restaurant is partnered with DoorDash or whoever because why wouldn't they. All the restaurant's IP (menu, photos, logos) was stolen from their website and used without permission. The customer then thinks "this restaurant is trying to rip me off! They have one price on the website and then ask for more after I've placed my order." Cue the Twitter outrage, complaint calls, bad mouthing to friends, etc... DoorDash is controlling the narrative because they're the interface the customer has with the restaurant.