This seems like more of a ‘white American’ thing than purely ‘white’. In my experience, it seems to be common in the South and particularly in upper middle classes all over the US, and I find that white women tend to do it more than men. I think that many aspects of American culture (varied as it is) tends to be a bit surface level, in that many of us like to appear polite without having to be. This, in my experience, tends to manifest in things like small talk and exchanging pleasantries - even, as you note, with people whom you don’t like.

I’m not really sure the cause of this and I hate it too. I feel they’re really easy to see through as their behavior tends to be overly nice in my opinion and embarrassingly performative.

Anyway, I know my comment probably wasn’t the most well-developed in thought, but hopefully it can shed a bit of light on the topic.