It’s still only one product. The innards and the software are exactly the same, no matter which watch you buy. Functionally they are all identical.
The customisability of the exterior is something new, sure.
Don't forget, the iMac G3 had five plastic color cases for some reason. Apple also sells a range of iPhone/iPad cases. Also, a major early selling point on the iPad was the magnates to better integrate with a case.
"for some reason"
Yeah, cars should only come in one color, right?
The MacBook Air comes in one colour, as do nearly all of their current products. Sure you can put it in a case, or put another sticker on it.