People really over-exaggerate the claim of friendly and polite US service workers and people in general. Obviously you can find the full spectrum of character types across the US. I've lived 2/3 of my life in Britain and 1/3 in the US and I honestly don't think there's much difference in interactions day to day. If anything I mostly just find Britain to be overly pessimistic and gloomy now.

Britain, or at the very least England, is also well known for its extreme politeness culture. Also, it's not that the US has a culture of genuine politeness, just a facade of it.

I have only spent about a year in the US, but to me the difference was stark from what I'm used to in Europe. As an example, I've never encountered a single shop cashier who didn't talk to me. Everyone had something to say, usually a variation of How's it going?. Contrasting this to my native Estonia, where I'd say at least 90% of my interactions with cashiers involves them not making a single sound. Not even in response to me saying hello, or to state the total sum. If they're depressed or in an otherwise non-euphoric mood, they make no attempt to fake it. I'm personally fine with it, because I don't go looking for social connections from cashiers. Also, when they do talk to me in a happy manner, I know it's genuine.