As a Swede, I think the reason this baffles you has a lot to do with the fact that the U.K. is comparatively tiny, with 67 mil inhabitants on 244 sqkm. Sweden for example has 10.5mil inhabitants on 450 sqkm.
What happens is that densely populated areas will have access to these services, perhaps not for free, but they're at least there. Less sparsely populated areas have less service coverage, and so you get more car dependant. Here in Sweden at least we have a decent public transport network so even in my old village of 600 people you could make do without a car, you just couldn't be particularly spontaneous about things.
The U.S. is very much structured around owning cars. Massive roads, poor pedestrian/cycling infrastructure, and a general lack of public transit. I visited Massachusetts back in 2019 and got a completely different perspective on things. Until then I didn't understand why my friend just didn't bike everywhere, but having been there it's easy to see that it's not viable. Even the cul-de-sac they lived on wasn't very pedestrian friendly.
That's not to say that the U.S. could have more sensible sizes on their cars, they definitely could. I think the sizes of cars growing has to do with manufacturers wanting increased profit. We're seeing an increase in the average car size here in Europe as well, with a lot of the more compact cars being taken off the market.