(Context clarification: I'm not an employee of a Canadian tech company; I'm a cofounder + major shareholder of one. I'm looking at this from the hiring side, not the being-hired side.)
Wages might be "converging" to the same numeric value — CAD$N in Canada vs USD$N in the US — but those are, and have always been, very different amounts of money. The two countries are still nowhere near the sort of economic equilibrium-state where Canadian companies are paying the prevailing US salary exchanged into Canadian dollars.
Also, there are a number of costs related to hiring US remote workers (as a company headquartered outside the US) that don't apply to hiring remote workers in most other countries. You're expected to provide some kind of private insurance plan, for example. Take-home pay may be converging, but the cost to the company of hiring a salaried US employee — or even a US contractor, as long as you care about being equitable / not building resentment when your employees talk amongst themselves about relative total compensation — is still higher than the cost to the company of hiring elsewhere.
And to be clear, the prevailing wage isn't even that relevant, as we're not really talking about "hiring the median American" vs "hiring the median [person living somewhere else]." We're talking about hiring people who apply for — and are considered top candidates for — remote tech jobs, among a global candidate pool. In the US, these people mostly happen to live in HCOL areas (like the Bay Area); which means their BATNA is to be paid far more than the US median wage for an SWE, because that's what the companies in HCOL areas have to offer to retain local talent there. To hire these people, you can't just offer "the going rate"; you have to offer FAANG money.