The simpler explanation is that there is no middle man demanding a 15%+ cut of the menu price. https://www.restaurantbusinessonline.com/technology/uber-eat...

Restaurant margins are already thin. So they hike the prices to pay UberEats and co. their share.

People definitely order appetizers and drinks, though. And lets not forget that while some of these take less effort to cook, they take a bit more effort in other ways. Cost in cleanup and wait staff that you don't have with takeout, for example.

Also, bigger margin doesn't always mean less effort to cook. It might depending on labor costs and the cost of goods (gravy mix is more expensive than the ingredients, but takes a bit more time to prepare) and you might also wind up needing consistency: Waffle House (a chain restaurant) probably wants its gravy to taste the same in two different cities, and it doesn't matter so much if it takes away a bit of margin.