No, you're right. Many people on HN would scoff at UberEats taking a 30% cut or whatever from orders, but how much would it cost for every mom and pop to try and implement something like that themselves? And have someone to maintain it? Whatever people want to say about it, these are orders (or at least a signficant number of them) that the restaurants would've never received, if not for the app existing. 70% is better than 0%.
And that 30% isn't in stone. Last place I worked at, they were able to negotiate for 15% because of how desperate Uber was.