Do we even have concrete numbers on how many got sold?
Anyway, the reason is probably because the games are not exchangeable, meaning for another game you'd need to buy another G&W.
And if one really would want to be stingy like this, where are the Color TV Game 7 and 15?
And i agree, its not the same because Game & Watch were multiple devices that only played a single game each so it belongs in a different category.
When the herd loses its way, the shepard must kill the bull that leads them astray.