For some reason Apple does not allow third party developers to create watch faces for Apple Watch. However, this may change in the near future if the watchOS 4.3.1 code is to be believed. The data from the code indicates that in the future Apple Watch they will start supporting third-party watch faces.
Most likely, support will appear with the release of watchOS 5, which will be presented at the World Developers Conference 2018 in a couple of months.
The NanoTimeKit Watch Faces component contains developer tools for working with Xcode on the Mac. The code looks like this:
Apple likes to severely restrict their operating systems and therefore the company has only allowed support for third-party watch faces now. However, with the release of watchOS 5, there should be plenty of varied and high quality watch faces for Apple Watch.
Third party watch faces will allow owners Apple Watch to customize them to their liking. Currently Apple offers about 15 watch faces. This is already a lot, but for all this time, they probably bored users, and a greater choice will only please them.