I'm actually taking issue with that assertion - mid-sized builders absolutely don't have that kind of time put into the designs of their house. They build one and iterate on the problems - it's not so different from software. I know - we've had quite a few problems related to being the 2nd iteration of a new house plan for a builder.

I think the "good house" would need to be defined first.

Like, going by objective measures like "how well it is insulated and how much it costs to cool/heat it", or "how well it uses the space of the plot" most of them fall well within "good", partly because at least on insulation level most countries require them to be at least decent.

But how well that fits the new owners ? Now that's where there would be actual benefit from either customization or doing it from scratch.