The problem is that most people don’t have a “Doordash or gtfo” attitude. They don’t mind doing take out or eating in on their own, but sometimes they just want something delivered. So they go onto Doordash and browse for places to try, see restuarant X and order from it.
The problem is that if restaurant X hasn’t agreed to be on Door dash, there are ramifications for what happens. This means the menu isn’t always up to date, the food may not be something that’s easy to transport in good condition (and optimal temperature), there’s less coordination, and the prices are increased without indication that they are Doordash specific prices.
This has caused people to have bad experiences, but since there’s no difference between “restaurant working with doordash” and “restaurant not working with doordash” consumers assume that that’s just the way the restaurant is. This has caused people to call into restaurants and complain (even though they ordered through DD) and made people dislike the restaurant, not door dash’s delivery. There is considerable anectdotes from business owners.
So yes, they’d rather the people who aren’t “DoorDash or nothing” not have their experience taken out of their control without their consent, and presented like they are an official Doordash partner.