Being specific has disadvantages, but the big advantage is that most decisions can be made by contractors reading the code instead of requiring an (expensive) engineering calculation. Engineering can be 1/3 of the cost of a remodel as it is. Increasing the engineering cost gives a big advantage to tract home builders who can amortize the engineering across a subdivision, which results in suburban malaise.
For instance, current energy codes require a certain R-value of insulation in walls, so that any contractor can go to Home Depot and buy the insulation marked with that R value and expect it to pass inspection. If the code said "Homes may only lose X BTU/year through all surfaces" it'd require a lot of engineer time per house to work it all out.
It's great that solar has evolved from being so expensive that it could only be installed in the most carefully optimized system, to something where you can just slap a few panels on a roof and expect reasonable results.
To reduce transmission losses, among other reasons.
> California has vast desert areas to tile with solar panels at less expense.
Even if it would be less build/maintenance expense per unit of delivered energy, a lot of California's vast deserts are either federally protected, and so not available even if it was smart to use them otherwise or already being irrigated and used for agriculture. You can't really cover a farm with solar panels without killing it's function like you can a house.
It's true that they more frequently get modified, often repeatedly and radically, until little trace of the original remains.
But anyhow, this isn't a law it's a regulation, and regulations are somewhat more likely to be removed (sometimes, because removal or even modification of the law under which the regulation was adopted forces this, independent of what the regulator would choose without that constraint.)