The deal used to be that it cost less to live here, so the lower pay was mostly made up for. Now it doesn't because housing costs hyper-inflated under low interest rates, lax regulation, and high immigration.

That said, while senior SWE salaries here absolutely not even close to competitive with the Bay Area or NYC they are not that far off from other places in the US (like smaller midwest cities etc). While Canadian salary packages are depressed, it's also partially a result of: they're not inflated.

The bigger concern is more the lack of interesting work.

Yep. In the mid-2000s when we first got married and had young kids my wife worked for Apple Canada, and I worked for a company that was just getting acquired by Google. We could easily have just both company-transferred to the Bay Area and made 3x the money and had much nicer weather. My mother-in-law had just passed away (fuck Cancer, etc) and there was little tying us here family wise. But choices were made: Canadian public school system vs US, being a citizen and secure in that here (vs insecure visa status), I wanted a rural property and to live rural (which would not have been a viable option there), etc. etc. For better or for worse we stayed here.

Many do similar calculations. Home is home. It's profoundly arrogant (and very American) to assume that the "best talent" moves for money and opportunity.

Americans have very high labour mobility rates, and often assume that's how the whole world works. But in most places in the world the idea of uprooting your whole life for a job is seen as odd.