Some companies do. But there are some benefits and relationships you can only get from having regular face-to-face interactions. These benefits aren't adamantly necessary and some companies are willing to trade them off for access to a wider labor pool, but face to face communication is a big enough part of American business culture that it's still the norm.
SF in particular is kind of a monoculture which could also help explain why those companies hire from within SF and the US.
You might be willing to take a flight every week and work from home, but as labor you also could be in the minority. I like having an office to go to every day to get out of my house, and to see people, and I don't like travel. (Unless it's for fun.) Some people are fine with this arrangement, but again if most labor finds it suboptimal that's also why it's not a big part of our culture.
If a company is going to make the extra effort to have remote labor they're likely to hire from overseas to take advantage of lower rates. That might explain why you have less luck finding remote work from Vancouver.