Your assumption being poorly built houses from long ago haven't survived due to some flaw in construction. I'm not so convinced. 80 year old barns cobbled together from rough planks with no foundation to speak of aren't exactly rare in this corner of the world (Southeast US), and I'm pretty confident folks paid more attention to the construction of their homes than their barn. In population dense areas old houses have a tendency to get torn down to make way for new construction.