Amusing to read this as I’m in my 50s and my wife’s in her 60s. Along with almost everyone in the UK we drive manual, and in very narrow and steep lanes like your aunt. Reversing for a long distance when you meet another vehicle is normal - most roads here (Devon) were made from sheep tracks and follow contours. We both also drive larger vehicles like a 3.5 ton horse lorry and big trailers around here. I say it’s amusing because… everyone does it, it’s normal.

As for why - I think it’s because Americans (and maybe Canadians?) always have had cheap oil so have adopted very large engined cars. Automatic gearboxes on small engines like we had, especially before the 80s, are painful - always hunting for the right gear and tend to be uneconomic and high revving. Drive a 1 litre mini automatic from the 1970s and it’s just horrible compared to manual.