Catering requires extra dry and cold storage. From food containers to displays, extra supplies, utensils and equipment, a foodservice operator needs the extra space to store these pieces. Additional cold storage is necessary as well as blast chilling equipment and prep space for food prepared for the event.
Instead of having to have a venue that allows for the preparation of meals, you can get away with renting a cheaper place that doesn't have as many amenities. Therefore, you're not having to pay for a lot of “extras” certain places might have, such as stoves, fridges, dishwashers, etc.