Everyone have their reasons and circumstances. Any US salary is way higher than Canada, pick any field. That doesnot mean every Canadian has just one goal in life....to move to US and earn in USD.
Tell you about real cases, there are people in Canada who have priority dates in 2008 and 2009, who can get GC as soon as they can get perm done or even file through their old employer no perm needed, still they have chosen to stay in Canada, they are that point of life where they don't want any immigration hassel or waiting for kids to go to university and need to maintain residency status.