It was just policy decisions in many cases.
Quite a few:
https://en.wikipedia.org/wiki/Suburb#Post-war_suburban_expan...
Well, to be fair, if you are talking about a specific city in Europe then you should do the same here in the USA. London's american equivalent is NYC. You will not need a car here either.
I'm not sure this is a great test. (Though, yes, London does have a great transit system even though I've also taken cabs from time to time for various reasons.)
When I visit American cities and don't travel outside them, I very rarely rent a car and don't necessarily take cabs/Lyfts all that much other than getting from the airport. I'm usually staying/doing business in/eating in a relatively central location that often exists in cities that are otherwise fairly spread out.
To give just an example that I'm very familiar with. I never rent a car any longer when I visit Raleigh but I'd never claim you could live there without a car.
I recently came back from a visit to London, and did not once need a car (I did take an Uber because I wanted to see what it was like to drive from the left side!). It was fantastic being able to take the Underground anywhere I needed to be, but it was very clear that it was only possible because the city was already there and densely populated before the arrival of cars.