Why would they pay more?

As long as they can fill the seats they have open, paying "Canadian" rates, why not?

Sure, they are aware that some talent is leaving for greener pastures southward, but there is no "shortage" of talent available in the Vancouver market.

When/if enough of the developer supply has exited, that filling those seats becomes difficult, salaries will increase.

I wouldn't hold my breath.

Or maybe that Canadians are willing to work for less? (I remember reading an article that auto manufacturers charge more for the same cars in Canada simply because Canadians will pay more.)

Well, in the case of Toronto, I make a better salary relative to the cost of living (all things considered -- rent, health care, insurance) than I could make in SF or NY at the moment. Taxes in Cali aren't that different from Ontario, but the cost of living is nuts (and for the most part astonishingly suburban). Even if I could get slightly more money, I'd lose a lot of quality of life because my entire network is here. Personally, the idea of dropping my entire life for a pay cheque would necessitate a substantial increase in pay.

I think companies pay what the market will bear, and enough people feel the same way I do.