The absolute construction costs for a high-rise are a lot higher than for a low-rise 4 story apartment building. If I can't fill a high-rise in your city, why would I build it? I'm better off building the low-rise and selling-through.

Besides - SFH's predate most restrictive zoning laws (which really caught on in the 70's). SFH's, at the edges of the city, aren't going anywhere.