I just don't see why the heck they wouldn't do this.
Probably for all the similar reasons that led them to decide against selling Apple-branded TV sets. They don't want to be in the business of selling bulky commodity products where "Apple brand" adds limited value.
Not all is lost though. When they refresh 5K iMacs with TB3 - you might be able to use one in target display mode, as a dumb screen for your rMBP.