I’ll take a completely uneducated guess based purely from my own experience:
The second hand market for Apple Watches is much smaller than for phones. There are fewer people interested in the product, and fewer still that would be comfortable with a second hand piece of wearable tech.
Now, those people who perhaps want an affordable Apple Watch… They aren’t necessarily looking for a premium model. They want cheap. And you can get a brand new SE for £259 or a Series 8 for £419. They have the option of getting a nice strap later on.
Just my guess, based also on the fact that I bought a rather expensive Series 0 - which never worked well. 12 months later Apple released a model which worked so much better. From that point on, I myself only upgraded twice and each time went for the cheapest model available.