alt Hacker News7/11 is still 30-50% more expensive than the supermarkets, so irrespective of how affluent people are, it's a poor choice.
I used 'exchange rate' because not only is the yen weak, but the USD seems pretty strong - I guess it depends on where in the US you are from, but as a Brit, US feels expensive to me, Japan feels cheap, ergo Americans must find Japan even cheaper than I do.
I think tourists just don't know the names of grocery stores, don't see them on social media, and don't expect them to have essentially all the same items as convenience stores (and in even greater variety) because ones back home have disappointing prepared and packaged foods. They also just don't care about wasting money when it already feels cheap.
Unrelated but another tip unknown to tourists is to get cold drinks (even alcohol) at pharmacies instead of convenience stores: their beverage fridges tend to be set much colder.