Much of the post-fire development used unreinforced masonry and was built on poor-quality fill. We saw the effects of this during the 1989 quake in areas like the Marina district.
The preponderance of fill (and depth to bedrock) in the densest areas of SF is well-illustrated by the Millennium Tower fiasco[0]. The MT's pilings do not even reach bedrock!
[0] https://en.wikipedia.org/wiki/Millennium_Tower_(San_Francisc...
Also, the issue of some buildings being built to (allegedly) inadequate standards is a red herring. If that were the case, the city would be more than happy to have them condemned, and all manner of more useful structures (for affordable housing or otherwise, put in their place).
A comparatively tiny portion of the city actually. But anyway, this is far afield from the original topic of this thread.