The lack of immigration problems is one reason why the talent is less expensive. The other reason is probably because people in Canada get compensated in other ways. Many of my American friends who left Canada for employment eventually return when their kids reach school age because they can't stomach the harsh reality of the US education system. Or something happens to their health.
It's easy to knock Canada for its personal income tax rate, but compared to European alternatives, it's still quite low, despite offering a whole lot of baseline public goods.
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.