Custom apple watch faces

Wow, so it's possible to create an app that replaces the watchface. Why doesn't apple make an app and API to make this into part of their product? I'd even buy faces if they were treated like ringtones, where I have to pay a buck to get each one I like.

The one above is gorgeous and does a better job than any current official Apple options at being both information rich and minimalist at the same time.