and I think there's something different culturally about waiting in line for prepared food I'm going to eat there and waiting in line in the grocery store; I much more frequently am approached by random people in line at Sam's than at Safeway, even though I spend a lot more time in the latter.
If you mean "the coffee shop" (there's a long history there) or "the pub" (a longer history, in the western tradition there) sure, I agree. I'm not saying randos approach me all the time when I'm in those sorts of places, but it happens. It doesn't happen at the grocery store hardly at all.
It happens a lot more often in the ride sharing services than even the pub; Yesterday I took my Saturday afternoon ride to Fry's for sandwiches, (The Fry's Sunnyvale Cafe is passable. Not bad. Not as good as the dedicated food places on Murphy, say, but it is super nostalgic for me, so it's something of a weekend tradition.) and while nobody talked to me in the Fry's cafe (I mean, aside from the procurement of food. I have met other interesting people there in the past.) the driver actually apologized to me because he spent most of the ride there speaking with another rider in a foreign language, which was mostly interesting in that he seemed to think that I had some expectation of being included in conversation as he drove. But yeah, I talk to the drivers often, a lot more than talking to strangers in retail spaces.
I guess what I'm saying is that social spaces need space for idling. Sitting in the back of the car does this, as does serving food or drink. Buying groceries, I think, does not.