I, like most people I know, carry groceries on the bus. You get one of these[1] and use a little arm strength (or ask for help if you can't). Yes, sometimes we spend a lot of time (possibly hours) going to and from the grocery shop.
Depends where you are. Where I'm at, I can either drive to the grocery store in 15 minutes, or I can walk a mile to the nearest highway where I can make a fairly dangerous crossing to try flagging down a bus that comes about once per hour, with very little regularity and often fails to stop. Then the actual trip to the store, another brief walk, and I'm limited to what I can carry, so instead of going to the store about once or twice per week with no more than an hour of transit time, I'll probably be spending a solid 4-9 hours per week travelling to get groceries. One hour per trip walking from house to bus and back, half an hour on a bus, twenty minutes walking to the store and back, multiply by 2-3 times per week. Add an extra hour every time a bus is missed or doesn't stop, which would be at least once or twice per week.
And that's not even touching trying to get to work, which takes me about 15 minutes in a different direction, and I'm not aware of any public transport that would get me from here to there without routing me in some other direction first. I could probably take on a part time job in the time I'd spend walking or riding buses.
It just wouldn't make sense for the majority of people in my area.
