GrubHub's stock price isn't much different from where it was 5 years ago. UberEats isn't expected to turn a profit for another five years as they continue to lose money on every order.
There are no monopolies here taking advantage of restauranteurs. There's an entire ecosystem of food delivery apps competing with each other.
The reality of the situation is that delivering meals is extremely expensive. It doesn't have meaningful economies of scale the way package delivery does. Both restaurants and customers have to pay these huge delivery fees because that's just what delivery costs. (In fact, many are still being subsidized by massive VC.)
Running a restaurant is always a precarious business proposition in the first place. Adding delivery is another precarious choice -- you can massively increase revenue but only at the cost of massively decreasing profit percentage, and if you don't calculate exactly right it'll seem like a big waste-of-time wash in the end, or you'll even lose money. That's just how it works.
I have tons of sympathy for restaurant owners, but just because it's hard -- not because they're being taken advantage of, because they're not.
But laws that restrict the ability of food delivery apps to charge what the market will bear isn't going to solve anything. They don't have these extra profits lying around they can just moderately reduce. The inevitable result will be them dropping the restaurants that are least profitable for delivery entirely, and reducing deliverypeople so your food will take two or three times as long to deliver. That's just Econ 101 -- if your revenue is reduced to where there's no path to profit, you need to cut quality. End of story.