Marco Arment, Jason Snell and others have been complaining recently about Apple Watch faces saying they’re not good enough and Apple should allow 3rd party faces. I think the reason they don’t is the same reason they don’t allow UI customization (outside of accessability features) or themes on iOS. I’d also guess they don’t want to have to police a watch face store and get sued by real watch makers claiming copyright or trademark infringement. Even if one argues for a heavily curated store it’s still Apple doing the curating and then you’d just have disgruntled developers no one has ever heard of running to social media to complain about their watch face being rejected and the outrage mob livid with Apple and its walled garden. Arguing that Apple needs better faces is fine but I don’t see 3rd party faces happening anytime soon.