I am never going to question why they aren't able to do it; I only do these to suggest what it looks like if accomplished, and hope that they can make it a possibility.
I used Photoshop. They can't just slide a color gradient thingy over and voila! At least, that's what I heard they can't do. Obviously things are hex-coded, but every color is also material-dependent and light source-dependent. We're getting closer with things like grass, so hopefully over the course of time they can find the way to take care of these as well.
But again, I will never question their abilities of doing it. I just don't think it's that easy to do, and sometimes there
are restrictions based on the style guide. I don't think I've ever said that before, but I understand when excuses can be rather legitimate.