Correct it is advised by DD but eventually done by the merchant.
Customers are cheap and they're (partly) to blame. My theory is that Amazon conditioned people to view delivery as a free commodity and pizza places who had delivery baked into their model cemented it.
So if Doordash listed a delivery fee that covered their true cost of delivery, customers would balk. So they instead have to find creative ways to get enough. Maybe it's changed and Doordash cracked the secret, but when I'd looked into it years ago these companies barely got by — many of them actually losing money.