Northern Europe tends to have a mild climate in the places where people live. The Northern US is significantly south, yet gets significantly colder winters. There are places in Europe that get worse than the Northern US - but they are places where few people live and so not normally what you are talking about when talking about Europe.

Though good heat pumps are hard to find in the northern US. Most installers only know of gas furnace + A/C, and don't even try to install anything else. As you get farther south in the US heat pumps become common, but there it rarely gets much below freezing and so they don't need backup heating systems at all.