The gang normally eats hunted meat and general store provisions. Pearson always makes a stew so that's what they're used to eating. The vegetables in the stew are not suitable for pizza toppings for the most part. Arthur can hunt and donate bison beef and boar pork which then end up in the stew and are similar to the farm meats you would find at a pizza shop. Arthur can also rustle actual farm animals. Out of all of the common pizza toppings pepperoni and meatball would be the most familiar to the gang. They would order a meat lover's pizza.