I’m going to assume that Apple doesn’t trust developers to come up with good watch face designs.
IMO, the Apple Watch is a beautifully designed product, and maybe one of Apple’s best ever designs, so why ruin that with a bad watch face? Sure, there could be a some great designs, but the average consumer probably doesn’t really know what a good watch face looks like.
Some of, in fact a lot of, the Gear S3 third-party face offerings are just putrid. I agree that they should open it up, but it would likely be heavily monitored. I doubt Apple wants a customer giving the Apple Watch advertisement on their wrist to other potential customers while rocking an ugly watch face that does not represent what Apple would want.