What's the point of a smart watch if you can't change the dial and face however you want? It's like when carriers wouldn't let you load your own ring tones on to your phone, because they didn't want to cannibalize their own ring tone sales.
It seems to be part of Apple's strategy, to focus on Out Of The Box experience, not on user configuration. And to the greater point? To make profit. The perfect product is not the point, nor is the perfectly satisfied customer.
While I fully believe that Apple should open up to third-party faces, I am quite content with the customization options that they offer now.
I've never seen one worn in real life.
Personally, they're a less cool version of an 80s swatch.
As for the reasons why they don't, my speculation is the following (I'm not defending them): They are worried about custom watch faces that might perform poorly or draw too much battery. Watch is a challenging platform to develop for as watchOS is very protective of its resources and won't hesitate to kill your app. But Apple probably doesn't want to kill the watch face and at the same time doesn't trust the developers to develop their watch faces to Apple's standard.
The point of a smart phone is not ringtones. The point of a smart phone is not even being a phone anymore. It's everything else you do with it.
The point of a smart watch is not cool watch faces.
You want skins/customization, which is fair. But you have to admit it's not core functionality.