I don't know if this is a factor, but isn't parking a big problem in big UK cities? If you wanted to make a grocery store run, you'd have to fret about parking and traffic.

Most American cities don't really have this problem

Correction - most American suburbs don't have this problem. Grocery stores in most major American metropolises (NYC, SF, Boston, DC) can't afford the real estate for parking lots.

Most Americans do not live in cities anything like NYC, SF, Boston, or DC. Even in DC, the District of Columbia is is a relatively small jurisdiction of (generously) 700,000 people in a region of over 5 million. Even District grocery stores generally have on-site parking--perhaps over or under the store--and it is often free.

So, no correction is in order and it is perfectly accurate to say "Most American cities don't really have this problem."

Most UK supermarkets have large out-of-town retail parks for exactly this reason - although the public appetite for this is declining.

Used to buy mine online but I got so sick of dealing with the websites (Tesco was by far the worst, ASDA a close second) I went back to doing a weekly shop and grabbing a taxi to get home.

Now since I work in town I use the local market for all the fresh stuff (meat, fish, veg) and a variety of smaller shops for household supplies, it's actually cheaper (less waste since I mostly buy on the day of use and the market is way cheaper for most stuff than supermarkets with better quality).