There are people going in the same areas, and if a newcomer goes into an established area, established by someone else, then they’re going to get very mad, maybe hostile.
Usually the brand owner will provide exclusive territories to different venders so they would have incentives to sink these costs. But this is a cash and carry business and it is difficult to verify territorial encroachments. Headquarters has a difficult time policing violations and it is up to individual drivers to protect their investments, sometimes resulting in broken bones. This was once explained to me by an economist friend who once sold ice cream for Good Humor in New York, which is pronounced "Gujummer" in New York.