If you live in Japan without a car you will probably end up taking a taxi from time to time. They are lined up outside the major train stations. Overall, relying on mass-transit probably is more pleasant there, and takes less lifestyle-modifications, but you still have less freedom than owning a car. And probably take longer to get around usually. Also in plenty of Japan, by the way, you definitely need a car to get by reasonably. It's not like countries in Europe where half the population lives in the capital city.

If your goal is to avoid any sort of exercise (and the "forced" exercise is one of the biggest reasons to go carless in my opinion) then yes large sections of US cities may seem inacccessible without a car. But if you like walking like I do (at least up to a half-mile or so) then you can live just fine in US cities without a car, in my experience. I've done it all over.