I would suspect that Amex is locked into contracts with its partners on the co-branded cards which mean that they cannot freely alter card features such as earn and redemption rates.
I keep checking the Amex site with some trepidation. The Ultimate Card has now changed from "not available" to not being there. (Unless anyone knows where it is hiding!) I can't help feeling that Amex will be keen to "enhance" it when they are able to.