Actually we don't even "know" how to build buildings anymore -- if you compare the standards of modern, throwaway buildings (as in, generic commercial or residential buildings; not skyscrapers or luxury condos) with those of the 1850s-1940s.
If you compare the standards of modern buildings with those that have survived from the 1850s-1940s there is considerable survivor bias in play. There were plenty of crappy buildings built in the late 1800s eg http://en.wikipedia.org/wiki/Industrial_Revolution#Housing
Quite so. Today's "throwaway" buildings are light-years beyond the throwaway buildings (tenement apartments and such-like) of the 1850s-1940s. Such buildings don't meet any modern fire or construction codes, which is why so few of them still exist in unmodified form anywhere in the developed world.
Actually I was thinking more in terms of aesthetics (and long-term durability) than things like fire safety.
Also, the survival rates of older buildings has to do with a zillion different factors -- from changes in taste (and ideology) to fate (wars and natural disasters) to the simple economic happenstance of the particular plot of land these structures were located on.
That is, the vast majority of buildings are torn down deliberately, to replace them with something bigger or different; to purge a neighborhood of undesirable residents; or to purge collective memory of an unfashionable sense of design, or of place -- rather than because they run into disrepair (or get burned down).
