There's a pragmatic reason for that. It's a lot easier to immigrate to Canada or Australia than to the US. For example, it usually takes more than 10 years in the US to go through the waiting process to get in through family sponsorship.
You may not like it and I personally would never want to emigrate to the US but the reality is that many people do want to go including boatloads of educated canadians who have a much better social network, less crime etc... Many professionals choose to leave because they want to make more money, pay less taxes, have access to resources they would never have in their own country.