What prevents you from just building more housing in all that extra space you have? Zoning laws and nimbyism.
That's the wrong way to look at the problem. Developers can already build in the hinterlands, but there's virtually no demand to do so because the economic activity is concentrated in the established cities. You also run into the Siberia problem where despite having massive tracts of "free/undeveloped" land in the taiga, actually inhabiting it is very expensive because of how brutal the weather can be. Unless you have something like a literal gold mine or oil well, the expense of heating the buildings and maintaining the equipment is not at all economically viable.
Vancouver may be the most constrained geographically but quite a lot of it looks suburban and could be built up if the political will existed.
Or, Canadians could choose to build nothing but single-family homes, but if they make that choice, they should own it and admit it is the real reason rather than blaming foreigners.
There is plenty of cheap, available housing in the hinterlands of Canada. And yet, nobody wants to move there. Why do you think that is? Prime locations are scarce no matter how much land mass you have.
Canada has a deep labor shortage in its urban areas, with plenty of pent-up demand for growth.
It's certainly true that if we must perpetually conserve each and every single family home in an urban area, then we'll remain housing-constrained. If that's what everyone wants to do, that's their right, but it's necessarily going to result in lower income and living standards for the country.