> But why does the high density housing have to be where the SFH are?

It depends on where said SFHs are. Cities need to eventually grow upwards! If we never allowed land-reuse Manhattan would still be 2 stories on the whole borough.

I live in Toronto and the harshest NIMBYs are adjacent to downtown and/or subway stops. They fight tooth and nail against even 6 story condominiums on the major high streets (Margret Atwood, who lives in a large SFH close to downtown was among them recently-ish). The ultimate result is that we have a handful of extremely dense clusters of condominiums centred around a few subway stops that then super-concentrate people and cause extreme crowding on transit, amenities, etc. It would be better if you had mid-rise multi-unit dwellings more spread out.

It's actually become an acute problem in so many ways. The people who own the SFHs tend be over 50 and childless. The result is that many schools in these neighbourhoods are at barely 20% capacity, while the condominiums (where most younger families can afford to live) are causing the schools near them to be overflowing with kids.

> This is how it's done in the rest of the world, certainly Korea, Japan, and China. I see this in England and France as well.

The countries broadly speaking default to higher density than North America for historical and economic reasons. They have their own issues, but the majority of Koreans live in high rises and Japan rebuilds their buildings constantly: https://www.theguardian.com/cities/2017/nov/16/japan-reusabl...