Taking a trip on public transit to pick up groceries (or any similar task) still consumes time and is not convenient. I used to live in Manhattan, go to the shop on foot, and carry my groceries home. I would absolutely buy as much food as I could carry and use, because it was 45 minutes out of my night that could be used for something else, whether I bought 4 days of food or a single head of lettuce. People don't generally joyride public transit unless they have no other options.
In fact, having lived both ways, I'd say you're far more incentivized to do this in a suburban setting with a car, since you're sitting in a private bubble with your own music and climate for every part of the trip except the store itself.
> People don't generally joyride public transit unless they have no other options.
I think this really depends of the network you are talking about. In some European cities, public transit networks are a blast to use. If frequencies are high enough, and modalities are well connected, you can "ride" the city as easily and naturally on a metro/tram/bus as you would have done by walking. I've took a lot (a damn lot) of bus/trams just because they were there, passing by, and allowed me to save 3 or 4 minutes (with my annual ticket, price was never my mind).
(But I'm lucky enough to live in one of the lowest CO2 emitting countries in terms of energy mix.)
Fair, not every city has public transit as "gritty" as NYC.
I do remember an UberPool trip I took. We got another passenger along the way. The driver ended up having to go 8 blocks out of our trip's path to pick up the passenger. She went 1 block and then got out. At least when you take a bus or tram out of convenience you are not being so wasteful.
In fact, having lived both ways, I'd say you're far more incentivized to do this in a suburban setting with a car, since you're sitting in a private bubble with your own music and climate for every part of the trip except the store itself.