groceries are traditionally thin margin so you can reduce overhead in the administrative, marketing, supply chain parts of the business. you also get bargaining power with producers. most packaged food producers are already "massive" and you generally see the same brands with only minor regional variations (like some different kinds of bread) from store to store across the country. they would also have a better ability to purchase from local producers, pushing out smaller grocers and making it harder for them to access the limited supply of local farms and creameries etc. basically all the advantages of a streamlined supply chain would happen here (instead of running two competing supply chains in the same region they can now run one which knows the stocking demands of all the stores). There are actually a lot of good reasons for the food business especially to have only one retail outlet, since it would reduce food waste from overbuying, and simplify consumer choice problems (i have to go to a few stores if i really want to get all the brands I like) except for the fact that it means inevitable monopolisitic pricing and ends up punishing producers because they have no other options for who to sell to.