while mutually benefitial, i can understand how a restaurant don't want to become a commodity in a platform they don't control.
The food delivery apps unify food delivery into a single platform from which the customer orders, and thus, every restaurant is now in heavy competition with every other restaurant directly. Customers comparing prices is the number one thing that decreases restaurant profits. Therefore, while the sales are up due to these apps, they cannot keep the same margins as before.
Being commoditized in someone else's platform is not a great business strategy.