I'm in my 40s and have never (and will never) own a car. So I chose to live within reasonable walking distance of places I need to go. I walk everywhere, and carry groceries in a backpack. I do these things without even thinking about it. I'm the same weight I was when I was 19. I wear the same size clothes. I don't bother with gym membership. The way I see it, it's just a different way to live. Kinda like when people say "I'm not gonna have kids because that's not what my life is gonna be about".
Now, this is only possible in a city that was built (at least) hundreds of years before cars. Having visited the US I can honestly say that some cities (not the ones like New York) are designed completely around the car. Trying to walk everywhere in a city like that is futile and you'll just hate life and everyone around you.
It may be implausible for most people, but there are probably some people for whom it is an option they haven't considered. I personally rent a car if I need to go out of the city, and walk or (rarely) bus at all other times. Very convenient for me.
Now, this is only possible in a city that was built (at least) hundreds of years before cars. Having visited the US I can honestly say that some cities (not the ones like New York) are designed completely around the car. Trying to walk everywhere in a city like that is futile and you'll just hate life and everyone around you.