wait is it because they’d typically relate cities to its state (ie Dallas, Texas) rather than its country (ie Paris, France)? And the UK’s subcountries are near enough to states?
There are still some forms that only list England, Scotland, Wales, and Northern Ireland as country options, no GB or UK. I've most recently encountered this with air travel at check in for some airlines.
Here in the Netherlands people als use "England" sometimes/often to mean all of the UK.
Why do people say Holland when they mean The Netherlands?
Ancient names die hard.
Canadians do it because there’s a London, Canada that has around 400,000 people (medium-sized by Canadian standards) and is home to a major university. Americans probably either picked it up from them or went to college there.