Because the human aspect of taste is part of the feedback mechanism you need. I also think that the labs historically invest in making their models good at things that accelerate AI research. And that in the early era of coding models, it was very clear that the model being able to write correct code would accelerate research. In a way that you can't really make the same case for design. Not that getting good at design isn't important, it's that it's not directly in that flywheel.
I also think that the labs historically invest in making their models good at things that accelerate AI research.