If you made the optimal headers for your engine, they would probably not fit very well in you car.
So they become a compromise.
In a regular 90 degree V8, with the "uneven" firing order they are not so sensitive to primary equal lengths actually. It is more important to just have them long enough. And the right size.
Atleast that is what I have read in most tests.
It is of course an myriad of possibilites whit sizes and stepped primarys and 4-2-1 vs 4-1, collectors and such.. And a price difference too..
If you would make tuned length headers for a 90 degree V8 it would be like 180 degree headers that "simulate" an 180 degree even fire V8, that makes the primary pipes very long and hard to fit, and that length would maybe(?) not be optimal.
I am no expert, but it seems like you could maybe compare it to an dual vs single-plane intake manifold difference. Where "untuned" headers make better overall power in a 90 degree V8 and tuned is more for a typical high RPM window.
Maybe...