A quote from a recent Apple job advert for an engineer to work on Mail:

We prefer to have engineers who can handle both the UI and lower level implementation details.

Hmm… well… that explains why the user interface stuff coming out of Apple has been steadily declining over the last few years.

I know that there are engineers who can handle both, but they are few and far between, and frankly I doubt that Apple has hired that many of them recently.