What a interesting question that is! Not all sandwich meat is round, but you can always put square pieces of cheese catty-corner to the meat so the sandwich looks pretty. Also, a lot of smoking skins are in a cylinder shape so the meat, when sliced is round.
There are basically just a couple of meats that come round...The reason why is because such as bologna, salami, sausage, liverworst,hotdogs, are placed in casings. These meats are hung on racks for a period of time. whether it's for curing or temperature change, it easier to handle. plus the fact that these meats are mixed products so pushing them through the casings is a more practical way of storing the meats properly.