Payment processing still applies, on every order, on the full amount. Delivery still costs money, and if it is fulfilled by the platform’s couriers that is another fee layered on top. The checkout still belongs to somebody else.
And the part nobody puts on the marketing page: the customer relationship. Who holds the order history, the email address, the ability to tell you we are opening early on Saturday. When that sits with a third party, a small restaurant is renting access to its own regulars.
We would rather just have your order. Directly.