Logistics, son, logistics. You can only carry so many groceries on a bus: which means more trips —> time and/or more $$ for multiple round trips. And then there’s the matter of perishables.
Baloney. City buses for the poor are very cheap, and either they are making money or they have plenty of time. Plenty of groceries can be taken on a bus.
Have you actually ever lived in a major city and taken a city bus?
Make those 15 kids carry some grocery bags.