So as a console designer, if you had a choice between a GPU from Company A or a GPU from Company B, and the GPU from A was 15% better overall for your targets than the GPU from B but your rival had already secured a GPU from Company A, you'd choose the inferior GPU just to have a different GPU? How's that going to help your product?
If one GPU hardware vendor is clearly superior to another, then naturally it would make sense to go with that vendor. But AMD and NVIDIA are both equally good GPU hardware vendors IMO.