If I lived in Boston, there's basically no way I'm getting to the mountains, north shore, Cape Cod, western suburbs/exurbs where a lot of people I know live, etc. without a car that I either own, rent, or someone else does. The city itself has a pretty good public transit system and there's even a pretty good commuter rail--but that's designed for coming into the city from suburban train stations with parking lots. So I'd be pretty much limited to the metro itself without a car in some form.
Which is fine for some people. They'll just naturally tend to avoid activities that are a pain to do in one way or another.
Yeah, my impression visiting the US has been that once you leave a big city, things become difficult to navigate alarmingly quickly. It's a bit different here; the suburbs and other cities and towns are usually fairly accessible via public transport.
I think that's generally true. While it's not like you can easily get to every corner of the British Isles or Western Europe generally without a car, there's a lot you can do out of Dublin or London with public transit and maybe the odd cab ride. Generally good as Boston's public transit system is, other than taking the train to New York City, flying somewhere, or taking a bus to someplace that is mostly not very interesting without car transportation on the other end, your options are pretty limited beyond what's covered by the greater metro transit system.
Which is fine for some people. They'll just naturally tend to avoid activities that are a pain to do in one way or another.