I have been researching that same question lately. Most of it comes down to: - Modern houses have much more systems installed. Think HVAC, electricity, hot/co

I have been researching that same question lately. Most of it comes down to: - Modern houses have much more systems installed. Think HVAC, electricity, hot/cold water, sewage, lightning, networking, etc. - Safety and quality requirements/expectations of houses have improved. Think thermal insulation, sound insulation, fire safety, roof load, wind resistance, etc. - Material quality and sustainability have improved. Think no more asbestos, less flammable materials, no more lead in paint and pipes, etc. These were all used because they were very cheap, safer alternatives are often more expensive. - Wages and safety of workers have improved (for them) significantly. - Material prices (delivered) have gone up significantly. Partially due to better wages and safety of workers in the supply chain, but also due to scarcity (for example: our population grows faster than trees).