Their reasons vary. Some don’t want their businesses perception of quality to be taken out of their control (delivering cold food, marking up items, poor substitutions). Some would prefer their staff service and build relationships with customers directly, instead of disinterested and frequently quite demanding runners. Some just straight up disagree with the practice of third party delivery.
I think that it’s pretty unambiguously reasonable to choose to not allow an unrelated business to operate inside of your physical storefront. I also think that maps onto digital services.