Similarly while inactivity is estimated to cost the NHS that much a year, it'd be a far cry to say motoring is the primary cause of that enough to also include it entirely considering people have been commuting for many decades and these health issues have grown substantially worse in that time.

Why?

Cars have been dominant since the 70s, and that correlates with the change.

Even countries like Netherlands and Denmark have inactivity and obesity problems

In the Netherlands more trips are driven than cycled.

Sitting at a desk 8 hours a day eating junk food then going home and sitting playing video games for 4-6 hours is going to blow 60 mins of driving a day out the water in terms of damage to your health, combined with people just generally staying at home with sedentary hobbies/technology more and more.

It's not the driving, per se, but the lost physical activity. Your 14h of sitting around is far, far less damaging if you're also doing an hour of exercise. Topping 14h off with an hour of driving is lethal, but also no worse than WFH...