Survivorship bias. Poorly built houses from long ago are no longer here, reminding us how poorly they were constructed, only the best examples survive. In 150 years, only the best houses built today will still be standing, and people then will be compare these houses with flimsy mass produced homes and say the same things.
Some bias, yes, but there are other things at work. When my house was built large beams, wide siding boards and clear wood for hardwood floors was readily available. As the supply of those materials shrank cheaper woods and the first engineered wood products showed up but they weren't particularly good. Builders were switching to aluminum windows, formica, carpeting and drywall but hadn't necessarily figured out how to build well with those materials.
Now we have higher quality engineered materials made from cheaper wood but they cost a lot more so people don't use them for mass construction.
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.
