Perhaps i am being synical but i suspect profit / ease of making has quite a bit to do with it. I think its quite a quick easy job to make essix retainers, i think they are vacuum formed. But they can probably charge just as much, if not more, as they have the ‘less noticable’ look that many non-enthusiasts like.
The suppliers win each way, especially as they would seem not to be long-lasting.
Anyway, they have no appeal to me. A big smooth plastic plate suits me fine!