My understanding is that Canadian FAANG offices are staffed mostly with those who cannot (and often will never get) a US visa, plus the odd local who doesn't want to move to the US for personal or family reasons.
Some are Canadian, born and raised, but many are immigrants to Canada. If you were to ask most of them, I think they'd say that they'd prefer to stay in Vancouver, despite the lower salaries. The alternative for them would be Seattle.
I've never lived in Seattle personally, but the tech workers I know who've lived in both Seattle and Vancouver have all said they prefer the lifestyle in Vancouver.