I agree with you. I just finished installing a solar system that covers 1.5x my needs in my Southen California house. 5.4kw. My system cost installed was $2.61/watt - with $7000 being labor from a contractor and $6300 the hardware and panels. And my payback will be 4.2 years based on the past 29 months of usage.

Let us assume that there is simply a hardware expense because the builders already have roofers and electricians. It’s such a fraction of a mouse fart (as mentioned) that going solar will essentially pay for itself after 2 years.

I can see builders charging a premium for the "electric car package" or the "double your consumption" service but something like 3W of production for each square foot per month would be amazing.