What utility does the solid lumber provide over the lightweight softwoods? What utility does the copper provide over plastic?

For lightweight softwood, I can at least think of the utility that it will be cheaper. And for plastic over copper plumbing, same thing, it will cost less, but it will also be more resistant to corrosion and easier to repair.

It serves the utility of not being able to hear people breathe on the other side of the home. Speaking as someone who grew up in old New England colonial houses, I'd say that if anyone wants to understand why Californians build so many tech products without privacy, one needn't look any further than the lack of privacy to which they grew accustomed in their homes.

That has exactly zero to do with the lumber. Nothing at all. In fact heavy dense lumber would transmit sound better than a light wood.

I have a new house built from this 'crap' wood you distain, and you can't hear people in the next room because it has sound deadening materials in the walls.

If your house has sound deadening material inside the interior walls, then you must have a custom built home. Almost all spec homes have nothing but air in those interior wall spaces.

If it is "definitely not true" as you claim, surely someone has done an analysis and published data on electrical, flooring, structural strength, etc. Lets see some of that. Data is easier to discuss than opinions.