Should add that this works both ways. Most Canadians who can, do end up leaving for the US since they can easily make 2 - 4X the income, even after CoL and insurance. There is a non-trivial amount of brain drain happening in Canada.
My wife and I didn't want to settle down in America, even if the money would have been that much better. Seattle is a great city, but it's still very American and that overall American culture never felt like home to us.
I was an expat in the US for many years, the story was the same in the mid-90s. Many try it, some stay, some come back. I still work for a US company but moved back.
The current political climate has a big impact on encouraging people to stay in Canada, especially with NAFTA and the H1B programs hanging by a thread. Imagine thousands of Canadians with TN status getting told they need to move back immediately (More likely 30-60 days after a status change application) upon withdrawal from NAFTA. It will be a chaotic brain drain in reverse.
Amazon, Microsoft, Google all have large Canadian outposts lately for a reason.
The political climate is much talked about but if anything the rate of brain drain has increased as more and more SV companies make an effort to recruit in Toronto. In the early 2000s the subset of US companies that recruited at all in Canada did it strictly at UW. Now all the majors have a campus presence at even mid-tier Canadian schools like UVic.
This is not to say transfers south are not done... but it's not the primary purpose of having a Canadian presence.