See, Russia has a population density of 8.4/km2 (vs 33km2 in US).
But that doesn't stop Russian cities from building public transport. Just like how empty acreage and dessert wouldn't stop US cities from building decent public transport.
What matters is not the average density of the country, but the layouts of the cities.
A good example is NYC. According to your statement it shouldn't have public transport because the average of the country is too sparsely populated, but that doesn't really matter. Just like how the empty tundra in Russia doesn't prevent Moscow or St Peterburg from building public transport.
One major difference I noticed from the USA is that it's usually, at least in principle possible to get from any point A to any point B in a Canadian city over about 20,000 people, by public transit, because even small towns often have public transit systems in Canada. (It might take a couple hours, though.) In many US cities, it's simply not possible because there is no public transit system.
After all, in every developed country, most people live in cities. Government policy and funding seems to be most of the difference.
While I certainly agree there are substantial bright spots in parts of Canada for transit investment, there's also big and worsening issues.
Rural inter-city transit has been getting drastically worse out in much of country.
VIA Rail has mostly been in a long-term downward spiral of cuts/"service suspensions" outside the main Ontario/Quebec corridor.
Inter-city buses aren't much better. Recent years have had Greyhound giving up and quitting Western Canada, and Saskatchewan shutting down their inter-city bus company. The result being limited patchwork of services not coming anywhere close to the kind of service that used to be provided.
https://www.theguardian.com/world/2018/oct/29/canada-greyhou...
https://edmontonjournal.com/opinion/columnists/opinion-one-y...
And of course Canada has one of the lowest population densities when you average the population across the swaths of uninhabited arctic. But 90% of Canadians live within 100 miles of the US border in a handful of cities.
Regardless, reducing rural car ownership in most places means reducing quality of life. It also reduces economic well being. People have fewer employment options, can't access supermarkets and such...
The whole approach of "make driving expensive" as the main policy vehicle is heartless... The actual way it works is that only wealthier people can drive, and that driving is actually important to people's lives.
It's a totally different proposition in London and in the country. If England (scotland & Wales are more sparse) becomes the netherlands great... but losing your car without gaining regular bus routes is a genuine loss. People need to get around.