By improving the consumer experience. Better optimizations of which couriers go where and by what routes, faster delivery times (and hence warmer food), menus and restaurant directories optimized to show you what you actually want, better delivery time estimation, no need to talk to a human or re-enter your details for each new restaurant, that sort of thing.
At Uber scale, you have people working on improving metrics, and those improvements translate across all the restaurants that exist across the world. "John's Chicken" won't hire their own guys to do A/B testing on which pictures of their food generate more sales.
The expected result of a monopoly are rising prices and at best indifferent service and quality.