"Waymo regularly takes its autonomous vehicles on winter road trips to test the cars in snowy environments. In 2017, it was Michigan.
This year, Waymo will hit multiple wintry locales, including Truckee, California; Upstate New York; and Michigan, from the Upper Peninsula to the metro Detroit area"
They testing the driving ability not the cars themselves.
New countries would require them to go all through the regulation approval and teaching the car to drive in different countries with different rules. The USA is massive and diverse. Might as well start there.
And Michigan and Upstate New York can get pretty gnarly in the winter, and they’re quite populated areas as well. They’re taxis, so it makes sense for them to test in higher population density areas.