> Only trivially small ones that we'd do a few times a week. There's still lots of physical activity that we do (eg walking around, carrying things like shopping, sex, etc).

Not everywhere — one of the major reasons fitness (and BMI) is correlated with walkability of a city is that in many typical US cities, you barely have to walk anymore, while in the more walkable cities driving is less possible, and you’ll walk or cycle much more, and usually use transit more (which means walking to/from the stops).

All this adds up.